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SubjectRe: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
Date
On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> 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 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).
>
> 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
>
> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
> 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(-)
>
> 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);

This callback might race with 'xen_blkbk_probe()'. The race could result in
__NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
'be' to the 'dev'. Please _don't merge_ this patch now!

I will do more test and share results. Meanwhile, if you have any opinion,
please let me know.


Thanks,
SeongJae Park

> +}
> +
> /* ** 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)
> --
> 2.17.1
>

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