Granting pages consumes backend system memory. In systems configured
with insufficient spare memory for those pages, it can cause a memory
pressure situation. However, finding the optimal amount of the spare
memory is challenging for large systems having dynamic resource
utilization patterns. Also, such a static configuration might lack
flexibility.
To mitigate such problems, this patchset adds a memory reclaim callback
to 'xenbus_driver' (patch 1) and use it to mitigate the problem in
'xen-blkback' (patch 2). The third patch is a trivial cleanup of
variable names.
Base Version
------------
This patch is based on v5.4. A complete tree is also available at my
public git repo:
https://github.com/sjp38/linux/tree/blkback_squeezing_v8
Patch History
-------------
Changes from v7
(https://lore.kernel.org/xen-devel/[email protected]/)
- Update sysfs-driver-xen-blkback for new parameter
(suggested by Roger Pau Monné)
- Use per-xen_blkif buffer_squeeze_end instead of global variable
(suggested by Roger Pau Monné)
Changes from v6
(https://lore.kernel.org/linux-block/[email protected]/)
- Remove more unnecessary prefixes (suggested by Roger Pau Monné)
- Constify a variable (suggested by Roger Pau Monné)
- Rename 'reclaim' into 'reclaim_memory' (suggested by Roger Pau Monné)
- More wordsmith of the commit message (suggested by Roger Pau Monné)
Changes from v5
(https://lore.kernel.org/linux-block/[email protected]/)
- Wordsmith the commit messages (suggested by Roger Pau Monné)
- Change the reclaim callback return type (suggested by Roger Pau Monné)
- Change the type of the blkback squeeze duration variable
(suggested by Roger Pau Monné)
- Add a patch for removal of unnecessary static variable name prefixes
(suggested by Roger Pau Monné)
- Fix checkpatch.pl warnings
Changes from v4
(https://lore.kernel.org/xen-devel/[email protected]/)
- Remove domain id parameter from the callback (suggested by Juergen Gross)
- Rename xen-blkback module parameter (suggested by Stefan Nuernburger)
Changes from v3
(https://lore.kernel.org/xen-devel/[email protected]/)
- Add general callback in xen_driver and use it (suggested by Juergen Gross)
Changes from v2
(https://lore.kernel.org/linux-block/[email protected])
- Rename the module parameter and variables for brevity
(aggressive shrinking -> squeezing)
Changes from v1
(https://lore.kernel.org/xen-devel/[email protected]/)
- Adjust the description to not use the term, `arbitrarily`
(suggested by Paul Durrant)
- Specify time unit of the duration in the parameter description,
(suggested by Maximilian Heyne)
- Change default aggressive shrinking duration from 1ms to 10ms
- Merge two patches into one single patch
SeongJae Park (3):
xenbus/backend: Add memory pressure handler callback
xen/blkback: Squeeze page pools if a memory pressure is detected
xen/blkback: Remove unnecessary static variable name prefixes
.../ABI/testing/sysfs-driver-xen-blkback | 9 +++
drivers/block/xen-blkback/blkback.c | 57 ++++++++++++-------
drivers/block/xen-blkback/common.h | 2 +
drivers/block/xen-blkback/xenbus.c | 11 +++-
drivers/xen/xenbus/xenbus_probe_backend.c | 32 +++++++++++
include/xen/xenbus.h | 1 +
6 files changed, 90 insertions(+), 22 deletions(-)
--
2.17.1
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 <[email protected]>
Signed-off-by: SeongJae Park <[email protected]>
---
.../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 <[email protected]>
+Description:
+ How long the block backend buffers release every free pages in
+ those under memory pressure. The time is in milliseconds.
+ 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");
+
+static unsigned long buffer_squeeze_end;
+
+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))
+ 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,
}
+void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
+{
+ struct backend_info *be = dev_get_drvdata(&dev->dev);
+
+ xen_blkbk_update_buffer_squeeze_end(be->blkif);
+}
+
+
/* ** Connection ** */
@@ -1115,7 +1123,8 @@ 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 = xen_blkbk_reclaim_memory,
};
int xen_blkif_xenbus_init(void)
--
2.17.1
A few of static variables in blkback have 'xen_blkif_' prefix, though it
is unnecessary for static variables. This commit removes such prefixes.
Reviewed-by: Roger Pau Monné <[email protected]>
Signed-off-by: SeongJae Park <[email protected]>
---
drivers/block/xen-blkback/blkback.c | 37 +++++++++++++----------------
1 file changed, 17 insertions(+), 20 deletions(-)
diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 26606c4896fd..85ff629a7546 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -62,8 +62,8 @@
* IO workloads.
*/
-static int xen_blkif_max_buffer_pages = 1024;
-module_param_named(max_buffer_pages, xen_blkif_max_buffer_pages, int, 0644);
+static int max_buffer_pages = 1024;
+module_param_named(max_buffer_pages, max_buffer_pages, int, 0644);
MODULE_PARM_DESC(max_buffer_pages,
"Maximum number of free pages to keep in each block backend buffer");
@@ -78,8 +78,8 @@ MODULE_PARM_DESC(max_buffer_pages,
* algorithm.
*/
-static int xen_blkif_max_pgrants = 1056;
-module_param_named(max_persistent_grants, xen_blkif_max_pgrants, int, 0644);
+static int max_pgrants = 1056;
+module_param_named(max_persistent_grants, max_pgrants, int, 0644);
MODULE_PARM_DESC(max_persistent_grants,
"Maximum number of grants to map persistently");
@@ -88,8 +88,8 @@ MODULE_PARM_DESC(max_persistent_grants,
* use. The time is in seconds, 0 means indefinitely long.
*/
-static unsigned int xen_blkif_pgrant_timeout = 60;
-module_param_named(persistent_grant_unused_seconds, xen_blkif_pgrant_timeout,
+static unsigned int pgrant_timeout = 60;
+module_param_named(persistent_grant_unused_seconds, pgrant_timeout,
uint, 0644);
MODULE_PARM_DESC(persistent_grant_unused_seconds,
"Time in seconds an unused persistent grant is allowed to "
@@ -137,9 +137,8 @@ module_param(log_stats, int, 0644);
static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt)
{
- return xen_blkif_pgrant_timeout &&
- (jiffies - persistent_gnt->last_used >=
- HZ * xen_blkif_pgrant_timeout);
+ return pgrant_timeout && (jiffies - persistent_gnt->last_used >=
+ HZ * pgrant_timeout);
}
/* Once a memory pressure is detected, squeeze free page pools for a while. */
@@ -249,7 +248,7 @@ static int add_persistent_gnt(struct xen_blkif_ring *ring,
struct persistent_gnt *this;
struct xen_blkif *blkif = ring->blkif;
- if (ring->persistent_gnt_c >= xen_blkif_max_pgrants) {
+ if (ring->persistent_gnt_c >= max_pgrants) {
if (!blkif->vbd.overflow_max_grants)
blkif->vbd.overflow_max_grants = 1;
return -EBUSY;
@@ -412,14 +411,13 @@ static void purge_persistent_gnt(struct xen_blkif_ring *ring)
goto out;
}
- if (ring->persistent_gnt_c < xen_blkif_max_pgrants ||
- (ring->persistent_gnt_c == xen_blkif_max_pgrants &&
+ if (ring->persistent_gnt_c < max_pgrants ||
+ (ring->persistent_gnt_c == max_pgrants &&
!ring->blkif->vbd.overflow_max_grants)) {
num_clean = 0;
} else {
- num_clean = (xen_blkif_max_pgrants / 100) * LRU_PERCENT_CLEAN;
- num_clean = ring->persistent_gnt_c - xen_blkif_max_pgrants +
- num_clean;
+ num_clean = (max_pgrants / 100) * LRU_PERCENT_CLEAN;
+ num_clean = ring->persistent_gnt_c - max_pgrants + num_clean;
num_clean = min(ring->persistent_gnt_c, num_clean);
pr_debug("Going to purge at least %u persistent grants\n",
num_clean);
@@ -614,8 +612,7 @@ static void print_stats(struct xen_blkif_ring *ring)
current->comm, ring->st_oo_req,
ring->st_rd_req, ring->st_wr_req,
ring->st_f_req, ring->st_ds_req,
- ring->persistent_gnt_c,
- xen_blkif_max_pgrants);
+ ring->persistent_gnt_c, max_pgrants);
ring->st_print = jiffies + msecs_to_jiffies(10 * 1000);
ring->st_rd_req = 0;
ring->st_wr_req = 0;
@@ -675,7 +672,7 @@ int xen_blkif_schedule(void *arg)
if (time_before(jiffies, buffer_squeeze_end))
shrink_free_pagepool(ring, 0);
else
- shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
+ shrink_free_pagepool(ring, max_buffer_pages);
if (log_stats && time_after(jiffies, ring->st_print))
print_stats(ring);
@@ -902,7 +899,7 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
continue;
}
if (use_persistent_gnts &&
- ring->persistent_gnt_c < xen_blkif_max_pgrants) {
+ ring->persistent_gnt_c < max_pgrants) {
/*
* We are using persistent grants, the grant is
* not mapped but we might have room for it.
@@ -929,7 +926,7 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
pages[seg_idx]->persistent_gnt = persistent_gnt;
pr_debug("grant %u added to the tree of persistent grants, using %u/%u\n",
persistent_gnt->gnt, ring->persistent_gnt_c,
- xen_blkif_max_pgrants);
+ max_pgrants);
goto next;
}
if (use_persistent_gnts && !blkif->vbd.overflow_max_grants) {
--
2.17.1
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 <[email protected]>
You should likely have dropped Juergen RB, since you made some
non-trivial changes.
> Signed-off-by: SeongJae Park <[email protected]>
> ---
> .../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 Monn? <[email protected]>
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.
On Fri, 13 Dec 2019 15:34:35 +0100 "Roger Pau Monné" <[email protected]> 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 <[email protected]>
>
> You should likely have dropped Juergen RB, since you made some
> non-trivial changes.
Yes, I will!
>
> > Signed-off-by: SeongJae Park <[email protected]>
> > ---
> > .../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 Monn� <[email protected]>
>
> 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."
Great, will change!
>
> > + 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.
Good point, I will!
>
> > +
> > +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.
Ooops, I made so dumb mistakes... Will fix it.
>
> > + 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.
Good suggestion!
>
> > +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.
I thought its a rule to use two newlines between functions here, but seems it
was just a trivial nit. Will fix and send next version soon!
Thanks,
SeongJae Park
>
> Thanks, Roger.
>