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[Xen-devel] [PATCH v6 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

Each `blkif` has a free pages pool for the grant mapping. The size of the pool starts from zero and be 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, if a system (maybe mistakenly) allows `blkfront` running guests to attach a large number of devices, the guests could cause a memory pressure in the `blkback` running guest by attaching a large number of block devices and inducing I/O. System administrators can avoid such problematic situations 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 the 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 a 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 a 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. We 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 use a simple `dd` command. Default Performance ------------------- [dom0]# echo 1024 > /sys/module/xen_blkback/parameters/max_buffer_pages [instance]$ for i in {1..5}; do dd if=/dev/zero of=file \ bs=4k count=$((256*512)); sync; done 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8827 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8781 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8737 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8702 s, 38.7 MB/s Worst-case Performance ---------------------- [dom0]# echo 0 > /sys/module/xen_blkback/parameters/max_buffer_pages [instance]$ for i in {1..5}; do dd if=/dev/zero of=file \ bs=4k count=$((256*512)); sync; done 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.878 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8746 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8786 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8749 s, 38.7 MB/s In short, even worst case squeezing makes no visible performance degradation on this test machine. I think this is due to the slow speed of the I/O devices I used. In other words, the additional page allocation overhead is hidden under the much slower I/O latency. Nevertheless, pleaset note that this is just a very simple and minimal test using a slow block device. On systems using fast block devices such as ramdisks or NVMe SSDs, the results could be very different. If you are in such cases, you should control the squeezing duration via the module parameter. Reviewed-by: Juergen Gross <jgross@xxxxxxxx> Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx> --- drivers/block/xen-blkback/blkback.c | 22 ++++++++++++++++++++-- drivers/block/xen-blkback/common.h | 1 + drivers/block/xen-blkback/xenbus.c | 3 ++- 3 files changed, 23 insertions(+), 3 deletions(-) diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c index fd1e19f1a49f..b493c306e84f 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_reclaim(struct xenbus_device *dev) +{ + 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..8a3195d2dca7 100644 --- a/drivers/block/xen-blkback/common.h +++ b/drivers/block/xen-blkback/common.h @@ -383,6 +383,7 @@ 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_reclaim(struct xenbus_device *dev); 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..b596c6e8b006 100644 --- a/drivers/block/xen-blkback/xenbus.c +++ b/drivers/block/xen-blkback/xenbus.c @@ -1115,7 +1115,8 @@ 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 = xen_blkbk_reclaim, }; int xen_blkif_xenbus_init(void) -- 2.17.1 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/mailman/listinfo/xen-devel

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