Re: [PATCH v8 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[Roger Pau Monné]

On Fri, Dec 13, 2019 at 01:02:10PM +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
> 
> Reviewed-by: Juergen Gross <jgross@xxxxxxxx>

You should likely have dropped Juergen RB, since you made some
non-trivial changes.

> Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
> ---
>  .../ABI/testing/sysfs-driver-xen-blkback      |  9 ++++++++
>  drivers/block/xen-blkback/blkback.c           | 22 +++++++++++++++++--
>  drivers/block/xen-blkback/common.h            |  2 ++
>  drivers/block/xen-blkback/xenbus.c            | 11 +++++++++-
>  4 files changed, 41 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> index 4e7babb3ba1f..a74a6d513c9f 100644
> --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
> +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> @@ -25,3 +25,12 @@ 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:        Roger Pau Monné <roger.pau@xxxxxxxxxx>

I think you should be the contact for this feature, you are the one
that implemented it :).

> +Description:
> +                How long the block backend buffers release every free pages in
> +                those under memory pressure.  The time is in milliseconds.

"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."

> +                The default is 10 milliseconds.
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index fd1e19f1a49f..26606c4896fd 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -142,6 +142,21 @@ static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt)
>  		HZ * xen_blkif_pgrant_timeout);
>  }
>  
> +/* 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");

I would place this in xenbus.c so that you don't need the
xen_blkbk_update_buffer_squeeze_end helper, and can just set
blkif->buffer_squeeze_end from xen_blkbk_reclaim_memory.

> +
> +static unsigned long buffer_squeeze_end;

This variable should be removed...

> +
> +void xen_blkbk_update_buffer_squeeze_end(struct xen_blkif *blkif)
> +{
> +	blkif->buffer_squeeze_end = jiffies +
> +		msecs_to_jiffies(buffer_squeeze_duration_ms);
> +}
> +
>  static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
>  {
>  	unsigned long flags;
> @@ -656,8 +671,11 @@ 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, buffer_squeeze_end))

... and this comparison needs to use blkif->buffer_squeeze_end
instead.

> +			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..ba653126177d 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 {
> @@ -383,6 +384,7 @@ irqreturn_t xen_blkif_be_int(int irq, void *dev_id);
>  int xen_blkif_schedule(void *arg);
>  int xen_blkif_purge_persistent(void *arg);
>  void xen_blkbk_free_caches(struct xen_blkif_ring *ring);
> +void xen_blkbk_update_buffer_squeeze_end(struct xen_blkif *blkif);
>  
>  int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,
>  			      struct backend_info *be, int state);
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index b90dbcd99c03..09fe6cb5c4ea 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -824,6 +824,14 @@ static void frontend_changed(struct xenbus_device *dev,
>  }
>  
>  

I would place the module_param_named instance here, so it's close as
possible to it's only user.

> +void xen_blkbk_reclaim_memory(struct xenbus_device *dev)

This can be static and drop the xen_blkbk prefix AFAICT.

> +{
> +	struct backend_info *be = dev_get_drvdata(&dev->dev);
> +
> +	xen_blkbk_update_buffer_squeeze_end(be->blkif);

Set blkif->buffer_squeeze_end here.

> +}
> +
> +

Extra newline.

Thanks, Roger.