[v9,2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
diff mbox series

Message ID 20191213153546.17425-3-sjpark@amazon.de
State New
Headers show
Series
  • xenbus/backend: Add a memory pressure handler callback
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Commit Message

SeongJae Park Dec. 13, 2019, 3:35 p.m. UTC
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          212    3,325

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           852     6,424
    10          212     3,325
    100         203     3,340

As expected, the memory pressure has decreased as the duration is
increased, but the reduction stopped 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).

For the I/O performance measurement, I run a simple `dd` command 5 times
as below and collect the 'MB/s' results.

    $ for i in {1..5}; do dd if=/dev/zero of=file \
                             bs=4k count=$((256*512)); sync; done

If the underlying block device is slow enough, the squeezing overhead
could be hidden.  For the reason, I do this test for both a slow block
device and a fast block device.  I use a popular cloud block storage
service, ebs[1] as a slow device and the ramdisk block device[2] for the
fast device.

The results are as below.  'max_pgs' represents the value of the
`blkback.max_buffer_pages` parameter.

On the slow block device
------------------------

    max_pgs   Min       Max       Median     Avg    Stddev
    0         38.7      45.8      38.7       40.12  3.1752165
    1024      38.7      45.8      38.7       40.12  3.1752165
    No difference proven at 95.0% confidence

On the fast block device
------------------------

    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://aws.amazon.com/ebs/
[2] https://www.kernel.org/doc/html/latest/admin-guide/blockdev/ramdisk.html

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 .../ABI/testing/sysfs-driver-xen-blkback      | 10 +++++++++
 drivers/block/xen-blkback/blkback.c           |  7 +++++--
 drivers/block/xen-blkback/common.h            |  1 +
 drivers/block/xen-blkback/xenbus.c            | 21 ++++++++++++++++++-
 4 files changed, 36 insertions(+), 3 deletions(-)

Comments

Roger Pau Monne Dec. 16, 2019, 9:37 a.m. UTC | #1
On Fri, Dec 13, 2019 at 03:35:44PM +0000, SeongJae Park wrote:
> 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          212    3,325
> 
> 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           852     6,424
>     10          212     3,325
>     100         203     3,340
> 
> As expected, the memory pressure has decreased as the duration is
> increased, but the reduction stopped 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).
> 
> For the I/O performance measurement, I run a simple `dd` command 5 times
> as below and collect the 'MB/s' results.
> 
>     $ for i in {1..5}; do dd if=/dev/zero of=file \
>                              bs=4k count=$((256*512)); sync; done
> 
> If the underlying block device is slow enough, the squeezing overhead
> could be hidden.  For the reason, I do this test for both a slow block
> device and a fast block device.  I use a popular cloud block storage
> service, ebs[1] as a slow device and the ramdisk block device[2] for the
> fast device.
> 
> The results are as below.  'max_pgs' represents the value of the
> `blkback.max_buffer_pages` parameter.
> 
> On the slow block device
> ------------------------
> 
>     max_pgs   Min       Max       Median     Avg    Stddev
>     0         38.7      45.8      38.7       40.12  3.1752165
>     1024      38.7      45.8      38.7       40.12  3.1752165
>     No difference proven at 95.0% confidence
> 
> On the fast block device
> ------------------------
> 
>     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://aws.amazon.com/ebs/
> [2] 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, Roger.
SeongJae Park Dec. 16, 2019, 10 a.m. UTC | #2
On Mon, 16 Dec 2019 10:37:55 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:

> On Fri, Dec 13, 2019 at 03:35:44PM +0000, SeongJae Park wrote:
> > 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          212    3,325
> > 
> > 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           852     6,424
> >     10          212     3,325
> >     100         203     3,340
> > 
> > As expected, the memory pressure has decreased as the duration is
> > increased, but the reduction stopped 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).
> > 
> > For the I/O performance measurement, I run a simple `dd` command 5 times
> > as below and collect the 'MB/s' results.
> > 
> >     $ for i in {1..5}; do dd if=/dev/zero of=file \
> >                              bs=4k count=$((256*512)); sync; done
> > 
> > If the underlying block device is slow enough, the squeezing overhead
> > could be hidden.  For the reason, I do this test for both a slow block
> > device and a fast block device.  I use a popular cloud block storage
> > service, ebs[1] as a slow device and the ramdisk block device[2] for the
> > fast device.
> > 
> > The results are as below.  'max_pgs' represents the value of the
> > `blkback.max_buffer_pages` parameter.
> > 
> > On the slow block device
> > ------------------------
> > 
> >     max_pgs   Min       Max       Median     Avg    Stddev
> >     0         38.7      45.8      38.7       40.12  3.1752165
> >     1024      38.7      45.8      38.7       40.12  3.1752165
> >     No difference proven at 95.0% confidence
> > 
> > On the fast block device
> > ------------------------
> > 
> >     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://aws.amazon.com/ebs/
> > [2] https://www.kernel.org/doc/html/latest/admin-guide/blockdev/ramdisk.html

I forgot to update this section.  It contains two evaluation results which has
no big difference and also describes one test in wrong way (it induced direct
IO to the ramdisk).  For example, I would like to update this section as below:

    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>

Appreciate for your reviews.  You made this patch much better!


Thanks,
SeongJae Park

> 
> Thanks, Roger.
>

Patch
diff mbox series

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..4f6ea4feca79 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -824,6 +824,24 @@  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);
+
+	be->blkif->buffer_squeeze_end = jiffies +
+		msecs_to_jiffies(buffer_squeeze_duration_ms);
+}
+
 /* ** Connection ** */
 
 
@@ -1115,7 +1133,8 @@  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)