| Message ID | 20190809124305.GQ18351@dhcp22.suse.cz (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RFC] mm: drop mark_page_access from the unmap path | expand |
On Fri, Aug 09, 2019 at 02:43:24PM +0200, Michal Hocko wrote: > On Tue 06-08-19 19:55:09, Minchan Kim wrote: > > On Wed, Jul 31, 2019 at 09:21:01AM +0200, Michal Hocko wrote: > > > On Wed 31-07-19 14:44:47, Minchan Kim wrote: > [...] > > > > As Nick mentioned in the description, without mark_page_accessed in > > > > zapping part, repeated mmap + touch + munmap never acticated the page > > > > while several read(2) calls easily promote it. > > > > > > And is this really a problem? If we refault the same page then the > > > refaults detection should catch it no? In other words is the above still > > > a problem these days? > > > > I admit we have been not fair for them because read(2) syscall pages are > > easily promoted regardless of zap timing unlike mmap-based pages. > > > > However, if we remove the mark_page_accessed in the zap_pte_range, it > > would make them more unfair in that read(2)-accessed pages are easily > > promoted while mmap-based page should go through refault to be promoted. > > I have really hard time to follow why an unmap special handling is > making the overall state more reasonable. > > Anyway, let me throw the patch for further discussion. Nick, Mel, > Johannes what do you think? > I won't be able to answer follow-ups to this for a while but here is some superficial thinking. Minimally, you should test PageReferenced before setting it like mark_page_accessed does to avoid unnecessary atomics. I know it wasn't done that way before but there is no harm in addressing it now. workingset_activation is necessarily expensive. It could speculatively lookup memcg outside the RCU read lock and only acquire it if there is something interesting to lookup. Probably not much help though. Note that losing the potential workingset_activation from the patch may have consequences if we are relying on refaults to fix this up. I'm undecided as to what degree it matters. That said, I do agree that the mark_page_accessed on page zapping may be overkill given that it can be a very expensive call if the page gets activated and it's potentially being called in the zap path at a high frequency. It's also not a function that is particularly easy to optimise if you want to cover all the cases that matter. It really would be preferably to have knowledge of a workload that really cares about the activations from mmap/touch/munmap. mark_page_accessed is a hint, it's known that there are gaps with it so we shouldn't pay too much of a cost on information that only might be useful. If the system is under no memory pressure because the workloads are tuned to fit in memory (e.g. database using direct IO) then mark_page_accessed is only cost. We could avoid marking it accessed entirely if PF_EXITING given that if a task is exiting, it's not a strong indication that the page is of any interest. Even if the page is heavily shared page and one user exits, the other users will keep it referenced and prevent reclaim anyway. The benefit is too marginal too. Given the definite cost of mark_page_accessed in this path and the main corner case being tasks that access pages via mmap/touch/munmap (which is insanely expensive if done at high frequency), I think it's reasonable to rely on SetPageReferenced giving the page another lap of the LRU in most cases (the obvious exception being CMA forcing reclaim). That opinion might change if there is a known example of a realistic workload that would suffer from the lack of explicit activations from teardown context.
On Fri, Aug 09, 2019 at 02:43:24PM +0200, Michal Hocko wrote: > On Tue 06-08-19 19:55:09, Minchan Kim wrote: > > On Wed, Jul 31, 2019 at 09:21:01AM +0200, Michal Hocko wrote: > > > On Wed 31-07-19 14:44:47, Minchan Kim wrote: > [...] > > > > As Nick mentioned in the description, without mark_page_accessed in > > > > zapping part, repeated mmap + touch + munmap never acticated the page > > > > while several read(2) calls easily promote it. > > > > > > And is this really a problem? If we refault the same page then the > > > refaults detection should catch it no? In other words is the above still > > > a problem these days? > > > > I admit we have been not fair for them because read(2) syscall pages are > > easily promoted regardless of zap timing unlike mmap-based pages. > > > > However, if we remove the mark_page_accessed in the zap_pte_range, it > > would make them more unfair in that read(2)-accessed pages are easily > > promoted while mmap-based page should go through refault to be promoted. > > I have really hard time to follow why an unmap special handling is > making the overall state more reasonable. > > Anyway, let me throw the patch for further discussion. Nick, Mel, > Johannes what do you think? > > From 3821c2e66347a2141358cabdc6224d9990276fec Mon Sep 17 00:00:00 2001 > From: Michal Hocko <mhocko@suse.com> > Date: Fri, 9 Aug 2019 14:29:59 +0200 > Subject: [PATCH] mm: drop mark_page_access from the unmap path > > Minchan has noticed that mark_page_access can take quite some time > during unmap: > : I had a time to benchmark it via adding some trace_printk hooks between > : pte_offset_map_lock and pte_unmap_unlock in zap_pte_range. The testing > : device is 2018 premium mobile device. > : > : I can get 2ms delay rather easily to release 2M(ie, 512 pages) when the > : task runs on little core even though it doesn't have any IPI and LRU > : lock contention. It's already too heavy. > : > : If I remove activate_page, 35-40% overhead of zap_pte_range is gone > : so most of overhead(about 0.7ms) comes from activate_page via > : mark_page_accessed. Thus, if there are LRU contention, that 0.7ms could > : accumulate up to several ms. > > bf3f3bc5e734 ("mm: don't mark_page_accessed in fault path") has replaced > SetPageReferenced by mark_page_accessed arguing that the former is not > sufficient when mark_page_accessed is removed from the fault path > because it doesn't promote page to the active list. It is true that a > page that is mapped by a single process might not get promoted even when > referenced if the reclaim checks it after the unmap but does that matter > that much? Can we cosider the page hot if there are no other > users? Moreover we do have workingset detection in place since then and > so a next refault would activate the page if it was really hot one. I do think the pages can be very hot. Think of short-lived executables and their libraries. Like shell commands. When they run a few times or periodically, they should be promoted to the active list and not have to compete with streaming IO on the inactive list - the PG_referenced doesn't really help them there, see page_check_references(). Maybe the refaults will be fine - but latency expectations around mapped page cache certainly are a lot higher than unmapped cache. So I'm a bit reluctant about this patch. If Minchan can be happy with the lock batching, I'd prefer that.
On Fri 09-08-19 14:34:24, Johannes Weiner wrote: > On Fri, Aug 09, 2019 at 02:43:24PM +0200, Michal Hocko wrote: > > On Tue 06-08-19 19:55:09, Minchan Kim wrote: > > > On Wed, Jul 31, 2019 at 09:21:01AM +0200, Michal Hocko wrote: > > > > On Wed 31-07-19 14:44:47, Minchan Kim wrote: > > [...] > > > > > As Nick mentioned in the description, without mark_page_accessed in > > > > > zapping part, repeated mmap + touch + munmap never acticated the page > > > > > while several read(2) calls easily promote it. > > > > > > > > And is this really a problem? If we refault the same page then the > > > > refaults detection should catch it no? In other words is the above still > > > > a problem these days? > > > > > > I admit we have been not fair for them because read(2) syscall pages are > > > easily promoted regardless of zap timing unlike mmap-based pages. > > > > > > However, if we remove the mark_page_accessed in the zap_pte_range, it > > > would make them more unfair in that read(2)-accessed pages are easily > > > promoted while mmap-based page should go through refault to be promoted. > > > > I have really hard time to follow why an unmap special handling is > > making the overall state more reasonable. > > > > Anyway, let me throw the patch for further discussion. Nick, Mel, > > Johannes what do you think? > > > > From 3821c2e66347a2141358cabdc6224d9990276fec Mon Sep 17 00:00:00 2001 > > From: Michal Hocko <mhocko@suse.com> > > Date: Fri, 9 Aug 2019 14:29:59 +0200 > > Subject: [PATCH] mm: drop mark_page_access from the unmap path > > > > Minchan has noticed that mark_page_access can take quite some time > > during unmap: > > : I had a time to benchmark it via adding some trace_printk hooks between > > : pte_offset_map_lock and pte_unmap_unlock in zap_pte_range. The testing > > : device is 2018 premium mobile device. > > : > > : I can get 2ms delay rather easily to release 2M(ie, 512 pages) when the > > : task runs on little core even though it doesn't have any IPI and LRU > > : lock contention. It's already too heavy. > > : > > : If I remove activate_page, 35-40% overhead of zap_pte_range is gone > > : so most of overhead(about 0.7ms) comes from activate_page via > > : mark_page_accessed. Thus, if there are LRU contention, that 0.7ms could > > : accumulate up to several ms. > > > > bf3f3bc5e734 ("mm: don't mark_page_accessed in fault path") has replaced > > SetPageReferenced by mark_page_accessed arguing that the former is not > > sufficient when mark_page_accessed is removed from the fault path > > because it doesn't promote page to the active list. It is true that a > > page that is mapped by a single process might not get promoted even when > > referenced if the reclaim checks it after the unmap but does that matter > > that much? Can we cosider the page hot if there are no other > > users? Moreover we do have workingset detection in place since then and > > so a next refault would activate the page if it was really hot one. > > I do think the pages can be very hot. Think of short-lived executables > and their libraries. Like shell commands. When they run a few times or > periodically, they should be promoted to the active list and not have > to compete with streaming IO on the inactive list - the PG_referenced > doesn't really help them there, see page_check_references(). Yeah, I am aware of that. We do rely on more processes to map the page which I've tried to explain in the changelog. Btw. can we promote PageReferenced pages with zero mapcount? I am throwing that more as an idea because I haven't really thought that through yet. > Maybe the refaults will be fine - but latency expectations around > mapped page cache certainly are a lot higher than unmapped cache. > > So I'm a bit reluctant about this patch. If Minchan can be happy with > the lock batching, I'd prefer that. Yes, it seems that the regular lock drop&relock helps in Minchan's case but this is a kind of change that might have other subtle side effects. E.g. will-it-scale has noticed a regression [1], likely because the critical section is shorter and the overal throughput of the operation decreases. Now, the w-i-s is an artificial benchmark so I wouldn't lose much sleep over it normally but we have already seen real regressions when the locking pattern has changed in the past so I would by a bit cautious. As I've said, this RFC is mostly to open a discussion. I would really like to weigh the overhead of mark_page_accessed and potential scenario when refaults would be visible in practice. I can imagine that a short lived statically linked applications have higher chance of being the only user unlike libraries which are often being mapped via several ptes. But the main problem to evaluate this is that there are many other external factors to trigger the worst case. [1] http://lkml.kernel.org/r/20190806070547.GA10123@xsang-OptiPlex-9020
On Mon, Aug 12, 2019 at 10:09:47AM +0200, Michal Hocko wrote: > On Fri 09-08-19 14:34:24, Johannes Weiner wrote: > > On Fri, Aug 09, 2019 at 02:43:24PM +0200, Michal Hocko wrote: > > > On Tue 06-08-19 19:55:09, Minchan Kim wrote: > > > > On Wed, Jul 31, 2019 at 09:21:01AM +0200, Michal Hocko wrote: > > > > > On Wed 31-07-19 14:44:47, Minchan Kim wrote: > > > [...] > > > > > > As Nick mentioned in the description, without mark_page_accessed in > > > > > > zapping part, repeated mmap + touch + munmap never acticated the page > > > > > > while several read(2) calls easily promote it. > > > > > > > > > > And is this really a problem? If we refault the same page then the > > > > > refaults detection should catch it no? In other words is the above still > > > > > a problem these days? > > > > > > > > I admit we have been not fair for them because read(2) syscall pages are > > > > easily promoted regardless of zap timing unlike mmap-based pages. > > > > > > > > However, if we remove the mark_page_accessed in the zap_pte_range, it > > > > would make them more unfair in that read(2)-accessed pages are easily > > > > promoted while mmap-based page should go through refault to be promoted. > > > > > > I have really hard time to follow why an unmap special handling is > > > making the overall state more reasonable. > > > > > > Anyway, let me throw the patch for further discussion. Nick, Mel, > > > Johannes what do you think? > > > > > > From 3821c2e66347a2141358cabdc6224d9990276fec Mon Sep 17 00:00:00 2001 > > > From: Michal Hocko <mhocko@suse.com> > > > Date: Fri, 9 Aug 2019 14:29:59 +0200 > > > Subject: [PATCH] mm: drop mark_page_access from the unmap path > > > > > > Minchan has noticed that mark_page_access can take quite some time > > > during unmap: > > > : I had a time to benchmark it via adding some trace_printk hooks between > > > : pte_offset_map_lock and pte_unmap_unlock in zap_pte_range. The testing > > > : device is 2018 premium mobile device. > > > : > > > : I can get 2ms delay rather easily to release 2M(ie, 512 pages) when the > > > : task runs on little core even though it doesn't have any IPI and LRU > > > : lock contention. It's already too heavy. > > > : > > > : If I remove activate_page, 35-40% overhead of zap_pte_range is gone > > > : so most of overhead(about 0.7ms) comes from activate_page via > > > : mark_page_accessed. Thus, if there are LRU contention, that 0.7ms could > > > : accumulate up to several ms. > > > > > > bf3f3bc5e734 ("mm: don't mark_page_accessed in fault path") has replaced > > > SetPageReferenced by mark_page_accessed arguing that the former is not > > > sufficient when mark_page_accessed is removed from the fault path > > > because it doesn't promote page to the active list. It is true that a > > > page that is mapped by a single process might not get promoted even when > > > referenced if the reclaim checks it after the unmap but does that matter > > > that much? Can we cosider the page hot if there are no other > > > users? Moreover we do have workingset detection in place since then and > > > so a next refault would activate the page if it was really hot one. > > > > I do think the pages can be very hot. Think of short-lived executables > > and their libraries. Like shell commands. When they run a few times or > > periodically, they should be promoted to the active list and not have > > to compete with streaming IO on the inactive list - the PG_referenced > > doesn't really help them there, see page_check_references(). > > Yeah, I am aware of that. We do rely on more processes to map the page > which I've tried to explain in the changelog. > > Btw. can we promote PageReferenced pages with zero mapcount? I am > throwing that more as an idea because I haven't really thought that > through yet. That flag implements a second-chance policy, see this commit: commit 645747462435d84c6c6a64269ed49cc3015f753d Author: Johannes Weiner <hannes@cmpxchg.org> Date: Fri Mar 5 13:42:22 2010 -0800 vmscan: detect mapped file pages used only once We had an application that would checksum large files using mmapped IO to avoid double buffering. The VM used to activate mapped cache directly, and it trashed the actual workingset. In response I added support for use-once mapped pages using this flag. SetPageReferenced signals the VM that we're not sure about the page yet and give it another round trip on the LRU. If you activate on this flag, it would restore the initial problem of use-once pages trashing the workingset. > > Maybe the refaults will be fine - but latency expectations around > > mapped page cache certainly are a lot higher than unmapped cache. > > > > So I'm a bit reluctant about this patch. If Minchan can be happy with > > the lock batching, I'd prefer that. > > Yes, it seems that the regular lock drop&relock helps in Minchan's case > but this is a kind of change that might have other subtle side effects. > E.g. will-it-scale has noticed a regression [1], likely because the > critical section is shorter and the overal throughput of the operation > decreases. Now, the w-i-s is an artificial benchmark so I wouldn't lose > much sleep over it normally but we have already seen real regressions > when the locking pattern has changed in the past so I would by a bit > cautious. I'm much more concerned about fundamentally changing the aging policy of mapped page cache then about the lock breaking scheme. With locking we worry about CPU effects; with aging we worry about additional IO. > As I've said, this RFC is mostly to open a discussion. I would really > like to weigh the overhead of mark_page_accessed and potential scenario > when refaults would be visible in practice. I can imagine that a short > lived statically linked applications have higher chance of being the > only user unlike libraries which are often being mapped via several > ptes. But the main problem to evaluate this is that there are many other > external factors to trigger the worst case. We can discuss the pros and cons, but ultimately we simply need to test it against real workloads to see if changing the promotion rules regresses the amount of paging we do in practice.
On Mon 12-08-19 11:07:25, Johannes Weiner wrote: > On Mon, Aug 12, 2019 at 10:09:47AM +0200, Michal Hocko wrote: [...] > > Btw. can we promote PageReferenced pages with zero mapcount? I am > > throwing that more as an idea because I haven't really thought that > > through yet. > > That flag implements a second-chance policy, see this commit: > > commit 645747462435d84c6c6a64269ed49cc3015f753d > Author: Johannes Weiner <hannes@cmpxchg.org> > Date: Fri Mar 5 13:42:22 2010 -0800 > > vmscan: detect mapped file pages used only once > > We had an application that would checksum large files using mmapped IO > to avoid double buffering. The VM used to activate mapped cache > directly, and it trashed the actual workingset. > > In response I added support for use-once mapped pages using this flag. > SetPageReferenced signals the VM that we're not sure about the page > yet and give it another round trip on the LRU. > > If you activate on this flag, it would restore the initial problem of > use-once pages trashing the workingset. You are right of course. I should have realized that! We really need another piece of information to store to the struct page or maybe xarray to reflect that. > > > Maybe the refaults will be fine - but latency expectations around > > > mapped page cache certainly are a lot higher than unmapped cache. > > > > > > So I'm a bit reluctant about this patch. If Minchan can be happy with > > > the lock batching, I'd prefer that. > > > > Yes, it seems that the regular lock drop&relock helps in Minchan's case > > but this is a kind of change that might have other subtle side effects. > > E.g. will-it-scale has noticed a regression [1], likely because the > > critical section is shorter and the overal throughput of the operation > > decreases. Now, the w-i-s is an artificial benchmark so I wouldn't lose > > much sleep over it normally but we have already seen real regressions > > when the locking pattern has changed in the past so I would by a bit > > cautious. > > I'm much more concerned about fundamentally changing the aging policy > of mapped page cache then about the lock breaking scheme. With locking > we worry about CPU effects; with aging we worry about additional IO. But the later is observable and debuggable little bit easier IMHO. People are quite used to watch for major faults from my experience as that is an easy metric to compare. > > As I've said, this RFC is mostly to open a discussion. I would really > > like to weigh the overhead of mark_page_accessed and potential scenario > > when refaults would be visible in practice. I can imagine that a short > > lived statically linked applications have higher chance of being the > > only user unlike libraries which are often being mapped via several > > ptes. But the main problem to evaluate this is that there are many other > > external factors to trigger the worst case. > > We can discuss the pros and cons, but ultimately we simply need to > test it against real workloads to see if changing the promotion rules > regresses the amount of paging we do in practice. Agreed. Do you see other option than to try it out and revert if we see regressions? We would get a workload description which would be helpful for future regression testing when touching this area. We can start slower and keep it in linux-next for a release cycle to catch any fallouts early. Thoughts?
On Tue 13-08-19 12:51:43, Michal Hocko wrote: > On Mon 12-08-19 11:07:25, Johannes Weiner wrote: > > On Mon, Aug 12, 2019 at 10:09:47AM +0200, Michal Hocko wrote: [...] > > > > Maybe the refaults will be fine - but latency expectations around > > > > mapped page cache certainly are a lot higher than unmapped cache. > > > > > > > > So I'm a bit reluctant about this patch. If Minchan can be happy with > > > > the lock batching, I'd prefer that. > > > > > > Yes, it seems that the regular lock drop&relock helps in Minchan's case > > > but this is a kind of change that might have other subtle side effects. > > > E.g. will-it-scale has noticed a regression [1], likely because the > > > critical section is shorter and the overal throughput of the operation > > > decreases. Now, the w-i-s is an artificial benchmark so I wouldn't lose > > > much sleep over it normally but we have already seen real regressions > > > when the locking pattern has changed in the past so I would by a bit > > > cautious. > > > > I'm much more concerned about fundamentally changing the aging policy > > of mapped page cache then about the lock breaking scheme. With locking > > we worry about CPU effects; with aging we worry about additional IO. > > But the later is observable and debuggable little bit easier IMHO. > People are quite used to watch for major faults from my experience > as that is an easy metric to compare. > > > > As I've said, this RFC is mostly to open a discussion. I would really > > > like to weigh the overhead of mark_page_accessed and potential scenario > > > when refaults would be visible in practice. I can imagine that a short > > > lived statically linked applications have higher chance of being the > > > only user unlike libraries which are often being mapped via several > > > ptes. But the main problem to evaluate this is that there are many other > > > external factors to trigger the worst case. > > > > We can discuss the pros and cons, but ultimately we simply need to > > test it against real workloads to see if changing the promotion rules > > regresses the amount of paging we do in practice. > > Agreed. Do you see other option than to try it out and revert if we see > regressions? We would get a workload description which would be helpful > for future regression testing when touching this area. We can start > slower and keep it in linux-next for a release cycle to catch any > fallouts early. > > Thoughts? ping...
On Mon, Aug 26, 2019 at 02:06:30PM +0200, Michal Hocko wrote: > On Tue 13-08-19 12:51:43, Michal Hocko wrote: > > On Mon 12-08-19 11:07:25, Johannes Weiner wrote: > > > On Mon, Aug 12, 2019 at 10:09:47AM +0200, Michal Hocko wrote: > [...] > > > > > Maybe the refaults will be fine - but latency expectations around > > > > > mapped page cache certainly are a lot higher than unmapped cache. > > > > > > > > > > So I'm a bit reluctant about this patch. If Minchan can be happy with > > > > > the lock batching, I'd prefer that. > > > > > > > > Yes, it seems that the regular lock drop&relock helps in Minchan's case > > > > but this is a kind of change that might have other subtle side effects. > > > > E.g. will-it-scale has noticed a regression [1], likely because the > > > > critical section is shorter and the overal throughput of the operation > > > > decreases. Now, the w-i-s is an artificial benchmark so I wouldn't lose > > > > much sleep over it normally but we have already seen real regressions > > > > when the locking pattern has changed in the past so I would by a bit > > > > cautious. > > > > > > I'm much more concerned about fundamentally changing the aging policy > > > of mapped page cache then about the lock breaking scheme. With locking > > > we worry about CPU effects; with aging we worry about additional IO. > > > > But the later is observable and debuggable little bit easier IMHO. > > People are quite used to watch for major faults from my experience > > as that is an easy metric to compare. Rootcausing additional (re)faults is really difficult. We're talking about a slight trend change in caching behavior in a sea of millions of pages. There could be so many factors causing this, and for most you have to patch debugging stuff into the kernel to rule them out. A CPU regression you can figure out with perf. > > > > As I've said, this RFC is mostly to open a discussion. I would really > > > > like to weigh the overhead of mark_page_accessed and potential scenario > > > > when refaults would be visible in practice. I can imagine that a short > > > > lived statically linked applications have higher chance of being the > > > > only user unlike libraries which are often being mapped via several > > > > ptes. But the main problem to evaluate this is that there are many other > > > > external factors to trigger the worst case. > > > > > > We can discuss the pros and cons, but ultimately we simply need to > > > test it against real workloads to see if changing the promotion rules > > > regresses the amount of paging we do in practice. > > > > Agreed. Do you see other option than to try it out and revert if we see > > regressions? We would get a workload description which would be helpful > > for future regression testing when touching this area. We can start > > slower and keep it in linux-next for a release cycle to catch any > > fallouts early. > > > > Thoughts? > > ping... Personally, I'm not convinced by this patch. I think it's a pretty drastic change in aging heuristics just to address a CPU overhead problem that has simpler, easier to verify, alternative solutions. It WOULD be great to clarify and improve the aging model for mapped cache, to make it a bit easier to reason about. But this patch does not really get there either. Instead of taking a serious look at mapped cache lifetime and usage scenarios, the changelog is more in "let's see what breaks if we take out this screw here" territory. So I'm afraid I don't think the patch & changelog in its current shape should go upstream.
On Tue 27-08-19 12:00:26, Johannes Weiner wrote: > On Mon, Aug 26, 2019 at 02:06:30PM +0200, Michal Hocko wrote: > > On Tue 13-08-19 12:51:43, Michal Hocko wrote: > > > On Mon 12-08-19 11:07:25, Johannes Weiner wrote: > > > > On Mon, Aug 12, 2019 at 10:09:47AM +0200, Michal Hocko wrote: > > [...] > > > > > > Maybe the refaults will be fine - but latency expectations around > > > > > > mapped page cache certainly are a lot higher than unmapped cache. > > > > > > > > > > > > So I'm a bit reluctant about this patch. If Minchan can be happy with > > > > > > the lock batching, I'd prefer that. > > > > > > > > > > Yes, it seems that the regular lock drop&relock helps in Minchan's case > > > > > but this is a kind of change that might have other subtle side effects. > > > > > E.g. will-it-scale has noticed a regression [1], likely because the > > > > > critical section is shorter and the overal throughput of the operation > > > > > decreases. Now, the w-i-s is an artificial benchmark so I wouldn't lose > > > > > much sleep over it normally but we have already seen real regressions > > > > > when the locking pattern has changed in the past so I would by a bit > > > > > cautious. > > > > > > > > I'm much more concerned about fundamentally changing the aging policy > > > > of mapped page cache then about the lock breaking scheme. With locking > > > > we worry about CPU effects; with aging we worry about additional IO. > > > > > > But the later is observable and debuggable little bit easier IMHO. > > > People are quite used to watch for major faults from my experience > > > as that is an easy metric to compare. > > Rootcausing additional (re)faults is really difficult. We're talking > about a slight trend change in caching behavior in a sea of millions > of pages. There could be so many factors causing this, and for most > you have to patch debugging stuff into the kernel to rule them out. > > A CPU regression you can figure out with perf. > > > > > > As I've said, this RFC is mostly to open a discussion. I would really > > > > > like to weigh the overhead of mark_page_accessed and potential scenario > > > > > when refaults would be visible in practice. I can imagine that a short > > > > > lived statically linked applications have higher chance of being the > > > > > only user unlike libraries which are often being mapped via several > > > > > ptes. But the main problem to evaluate this is that there are many other > > > > > external factors to trigger the worst case. > > > > > > > > We can discuss the pros and cons, but ultimately we simply need to > > > > test it against real workloads to see if changing the promotion rules > > > > regresses the amount of paging we do in practice. > > > > > > Agreed. Do you see other option than to try it out and revert if we see > > > regressions? We would get a workload description which would be helpful > > > for future regression testing when touching this area. We can start > > > slower and keep it in linux-next for a release cycle to catch any > > > fallouts early. > > > > > > Thoughts? > > > > ping... > > Personally, I'm not convinced by this patch. I think it's a pretty > drastic change in aging heuristics just to address a CPU overhead > problem that has simpler, easier to verify, alternative solutions. > > It WOULD be great to clarify and improve the aging model for mapped > cache, to make it a bit easier to reason about. I fully agree with this! Do you have any specific ideas? I am afraid I am unlikely to find time for a larger project that this sounds to be but maybe others will find this as a good fit. > But this patch does > not really get there either. Instead of taking a serious look at > mapped cache lifetime and usage scenarios, the changelog is more in > "let's see what breaks if we take out this screw here" territory. You know that I tend to be quite conservative. In this case I can see the cost which is not negligible and likely to hit many workloads because it is a common path. The immediate benefit is not really clear, though, at least to me. We can speculate and I would really love to hear from Nick what exactly led him to this change. > So I'm afraid I don't think the patch & changelog in its current shape > should go upstream. I will not insist of course but it would be really great to know and _document_ why we are doing this. I really hate how often we keep different heuristics and build more complex solutions on top just because nobody dares to change that. Our code is really hard to reason about.
diff --git a/mm/memory.c b/mm/memory.c index e2bb51b6242e..ced521df8ee7 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1053,7 +1053,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, } if (pte_young(ptent) && likely(!(vma->vm_flags & VM_SEQ_READ))) - mark_page_accessed(page); + SetPageReferenced(page); } rss[mm_counter(page)]--; page_remove_rmap(page, false);