| Message ID | 20191218183718.31719-4-sjpark@amazon.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | xenbus/backend: Add memory pressure handler callback | expand |
On Wed, Dec 18, 2019 at 07:37:16PM +0100, SeongJae Park wrote: > From: SeongJae Park <sjpark@amazon.de> > > Each `blkif` has a free pages pool for the grant mapping. The size of > the pool starts from zero and is increased on demand while processing > the I/O requests. If current I/O requests handling is finished or 100 > milliseconds has passed since last I/O requests handling, it checks and > shrinks the pool to not exceed the size limit, `max_buffer_pages`. > > Therefore, host administrators can cause memory pressure in blkback by > attaching a large number of block devices and inducing I/O. Such > problematic situations can be avoided by limiting the maximum number of > devices that can be attached, but finding the optimal limit is not so > easy. Improper set of the limit can results in memory pressure or a > resource underutilization. This commit avoids such problematic > situations by squeezing the pools (returns every free page in the pool > to the system) for a while (users can set this duration via a module > parameter) if memory pressure is detected. > > Discussions > =========== > > The `blkback`'s original shrinking mechanism returns only pages in the > pool which are not currently be used by `blkback` to the system. In > other words, the pages that are not mapped with granted pages. Because > this commit is changing only the shrink limit but still uses the same > freeing mechanism it does not touch pages which are currently mapping > grants. > > Once memory pressure is detected, this commit keeps the squeezing limit > for a user-specified time duration. The duration should be neither too > long nor too short. If it is too long, the squeezing incurring overhead > can reduce the I/O performance. If it is too short, `blkback` will not > free enough pages to reduce the memory pressure. This commit sets the > value as `10 milliseconds` by default because it is a short time in > terms of I/O while it is a long time in terms of memory operations. > Also, as the original shrinking mechanism works for at least every 100 > milliseconds, this could be a somewhat reasonable choice. I also tested > other durations (refer to the below section for more details) and > confirmed that 10 milliseconds is the one that works best with the test. > That said, the proper duration depends on actual configurations and > workloads. That's why this commit allows users to set the duration as a > module parameter. > > Memory Pressure Test > ==================== > > To show how this commit fixes the memory pressure situation well, I > configured a test environment on a xen-running virtualization system. > On the `blkfront` running guest instances, I attach a large number of > network-backed volume devices and induce I/O to those. Meanwhile, I > measure the number of pages that swapped in (pswpin) and out (pswpout) > on the `blkback` running guest. The test ran twice, once for the > `blkback` before this commit and once for that after this commit. As > shown below, this commit has dramatically reduced the memory pressure: > > pswpin pswpout > before 76,672 185,799 > after 867 3,967 > > Optimal Aggressive Shrinking Duration > ------------------------------------- > > To find a best squeezing duration, I repeated the test with three > different durations (1ms, 10ms, and 100ms). The results are as below: > > duration pswpin pswpout > 1 707 5,095 > 10 867 3,967 > 100 362 3,348 > > As expected, the memory pressure decreases as the duration increases, > but the reduction become slow from the `10ms`. Based on this results, I > chose the default duration as 10ms. > > Performance Overhead Test > ========================= > > This commit could incur I/O performance degradation under severe memory > pressure because the squeezing will require more page allocations per > I/O. To show the overhead, I artificially made a worst-case squeezing > situation and measured the I/O performance of a `blkfront` running > guest. > > For the artificial squeezing, I set the `blkback.max_buffer_pages` using > the `/sys/module/xen_blkback/parameters/max_buffer_pages` file. In this > test, I set the value to `1024` and `0`. The `1024` is the default > value. Setting the value as `0` is same to a situation doing the > squeezing always (worst-case). > > If the underlying block device is slow enough, the squeezing overhead > could be hidden. For the reason, I use a fast block device, namely the > rbd[1]: > > # xl block-attach guest phy:/dev/ram0 xvdb w > > For the I/O performance measurement, I run a simple `dd` command 5 times > directly to the device as below and collect the 'MB/s' results. > > $ for i in {1..5}; do dd if=/dev/zero of=/dev/xvdb \ > bs=4k count=$((256*512)); sync; done > > The results are as below. 'max_pgs' represents the value of the > `blkback.max_buffer_pages` parameter. > > max_pgs Min Max Median Avg Stddev > 0 417 423 420 419.4 2.5099801 > 1024 414 425 416 417.8 4.4384682 > No difference proven at 95.0% confidence > > In short, even worst case squeezing on ramdisk based fast block device > makes no visible performance degradation. Please note that this is just > a very simple and minimal test. On systems using super-fast block > devices and a special I/O workload, the results might be different. If > you have any doubt, test on your machine with your workload to find the > optimal squeezing duration for you. > > [1] https://www.kernel.org/doc/html/latest/admin-guide/blockdev/ramdisk.html > > Signed-off-by: SeongJae Park <sjpark@amazon.de> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com> Thanks, and sorry for the delay! Roger.
diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback index 4e7babb3ba1f..f01224231f3f 100644 --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback @@ -25,3 +25,13 @@ Description: allocated without being in use. The time is in seconds, 0 means indefinitely long. The default is 60 seconds. + +What: /sys/module/xen_blkback/parameters/buffer_squeeze_duration_ms +Date: December 2019 +KernelVersion: 5.5 +Contact: SeongJae Park <sjpark@amazon.de> +Description: + When memory pressure is reported to blkback this option + controls the duration in milliseconds that blkback will not + cache any page not backed by a grant mapping. + The default is 10ms. diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c index fd1e19f1a49f..79f677aeb5cc 100644 --- a/drivers/block/xen-blkback/blkback.c +++ b/drivers/block/xen-blkback/blkback.c @@ -656,8 +656,11 @@ int xen_blkif_schedule(void *arg) ring->next_lru = jiffies + msecs_to_jiffies(LRU_INTERVAL); } - /* Shrink if we have more than xen_blkif_max_buffer_pages */ - shrink_free_pagepool(ring, xen_blkif_max_buffer_pages); + /* Shrink the free pages pool if it is too large. */ + if (time_before(jiffies, blkif->buffer_squeeze_end)) + shrink_free_pagepool(ring, 0); + else + shrink_free_pagepool(ring, xen_blkif_max_buffer_pages); if (log_stats && time_after(jiffies, ring->st_print)) print_stats(ring); diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h index 1d3002d773f7..536c84f61fed 100644 --- a/drivers/block/xen-blkback/common.h +++ b/drivers/block/xen-blkback/common.h @@ -319,6 +319,7 @@ struct xen_blkif { /* All rings for this device. */ struct xen_blkif_ring *rings; unsigned int nr_rings; + unsigned long buffer_squeeze_end; }; struct seg_buf { diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c index b90dbcd99c03..24172c180f5f 100644 --- a/drivers/block/xen-blkback/xenbus.c +++ b/drivers/block/xen-blkback/xenbus.c @@ -824,6 +824,26 @@ static void frontend_changed(struct xenbus_device *dev, } +/* Once a memory pressure is detected, squeeze free page pools for a while. */ +static unsigned int buffer_squeeze_duration_ms = 10; +module_param_named(buffer_squeeze_duration_ms, + buffer_squeeze_duration_ms, int, 0644); +MODULE_PARM_DESC(buffer_squeeze_duration_ms, +"Duration in ms to squeeze pages buffer when a memory pressure is detected"); + +/* + * Callback received when the memory pressure is detected. + */ +static void reclaim_memory(struct xenbus_device *dev) +{ + struct backend_info *be = dev_get_drvdata(&dev->dev); + + if (!be) + return; + be->blkif->buffer_squeeze_end = jiffies + + msecs_to_jiffies(buffer_squeeze_duration_ms); +} + /* ** Connection ** */ @@ -1115,7 +1135,8 @@ static struct xenbus_driver xen_blkbk_driver = { .ids = xen_blkbk_ids, .probe = xen_blkbk_probe, .remove = xen_blkbk_remove, - .otherend_changed = frontend_changed + .otherend_changed = frontend_changed, + .reclaim_memory = reclaim_memory, }; int xen_blkif_xenbus_init(void)