| Message ID | 20191205150932.3793-2-sjpark@amazon.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | xen/blkback: Aggressively shrink page pools if a memory pressure is detected | expand |
On 05.12.19 16:09, SeongJae Park 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 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, `blkfront` running guests can 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 each guest can > attach. However, finding the optimal limit is not so easy. Improper > set of the limit can result in the memory pressure or a resource > underutilization. This commit avoids such problematic situations by > shrinking the pools aggressively (further the limit) 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`. In other words, the > pages that will be shrunk are not mapped with foreign pages. Because > this commit is changing only the shrink limit but uses the shrinking > mechanism as is, this commit does not introduce improper mappings > related security issues. > > Once a memory pressure is detected, this commit keeps the aggressive > shrinking limit for a user-specified time duration. The duration should > be neither too long nor too short. If it is too long, free pages pool > shrinking 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. That said, the proper duration depends on actual configurations > and workloads. That's why this commit is allowing users to set it as > their optimal value via the module parameter. > > Memory Pressure Test > ==================== > > To show how this commit fixes the above mentioned memory pressure > situation well, I configured a test environment on a xen-running 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 and out 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 aggressive shrinking 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 numbers have further decreased by increasing the > duration, 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 aggressive shrinking will require more page > allocations per I/O. To show the overhead, I artificially made an > aggressive pages pool shrinking situation and measured the I/O > performance of a `blkfront` running guest. > > For the artificial shrinking, 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 aggressive shrinking > 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 aggressive shrinking makes no visible > performance degradation. I think this is due to the slow speed of the > I/O. 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. > > Signed-off-by: SeongJae Park <sjpark@amazon.de> > --- > drivers/block/xen-blkback/blkback.c | 35 +++++++++++++++++++++++++++-- > 1 file changed, 33 insertions(+), 2 deletions(-) > > diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c > index 3666afa639d1..72d068328ef1 100644 > --- a/drivers/block/xen-blkback/blkback.c > +++ b/drivers/block/xen-blkback/blkback.c > @@ -135,6 +135,31 @@ module_param(log_stats, int, 0644); > /* Number of free pages to remove on each call to gnttab_free_pages */ > #define NUM_BATCH_FREE_PAGES 10 > > +/* > + * Once a memory pressure is detected, keep aggressive shrinking of the free > + * page pools for this time (milliseconds) > + */ > +static int xen_blkif_aggressive_shrinking_duration = 10; > +module_param_named(aggressive_shrinking_duration, > + xen_blkif_aggressive_shrinking_duration, int, 0644); > +MODULE_PARM_DESC(aggressive_shrinking_duration, > +"Duration in ms to do aggressive shrinking when a memory pressure is detected"); > + > +static unsigned long xen_blk_mem_pressure_end; > + > +static unsigned long blkif_shrink_count(struct shrinker *shrinker, > + struct shrink_control *sc) > +{ > + xen_blk_mem_pressure_end = jiffies + > + msecs_to_jiffies(xen_blkif_aggressive_shrinking_duration); > + return 0; > +} > + > +static struct shrinker blkif_shrinker = { > + .count_objects = blkif_shrink_count, > + .seeks = DEFAULT_SEEKS, > +}; > + > static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt) > { > return xen_blkif_pgrant_timeout && > @@ -656,8 +681,11 @@ 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, xen_blk_mem_pressure_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); > @@ -1500,6 +1528,9 @@ static int __init xen_blkif_init(void) > if (rc) > goto failed_init; > > + if (register_shrinker(&blkif_shrinker)) > + pr_warn("shrinker registration failed\n"); > + > failed_init: > return rc; > } CC-ing xen-devel@lists.xenproject.org Thanks, SeongJae Park
On 05.12.19 16:09, SeongJae Park 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 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, `blkfront` running guests can 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 each guest can > attach. However, finding the optimal limit is not so easy. Improper > set of the limit can result in the memory pressure or a resource > underutilization. This commit avoids such problematic situations by > shrinking the pools aggressively (further the limit) 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`. In other words, the > pages that will be shrunk are not mapped with foreign pages. Because > this commit is changing only the shrink limit but uses the shrinking > mechanism as is, this commit does not introduce improper mappings > related security issues. > > Once a memory pressure is detected, this commit keeps the aggressive > shrinking limit for a user-specified time duration. The duration should > be neither too long nor too short. If it is too long, free pages pool > shrinking 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. That said, the proper duration depends on actual configurations > and workloads. That's why this commit is allowing users to set it as > their optimal value via the module parameter. > > Memory Pressure Test > ==================== > > To show how this commit fixes the above mentioned memory pressure > situation well, I configured a test environment on a xen-running 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 and out 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 aggressive shrinking 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 numbers have further decreased by increasing the > duration, 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 aggressive shrinking will require more page > allocations per I/O. To show the overhead, I artificially made an > aggressive pages pool shrinking situation and measured the I/O > performance of a `blkfront` running guest. > > For the artificial shrinking, 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 aggressive shrinking > 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 aggressive shrinking makes no visible > performance degradation. I think this is due to the slow speed of the > I/O. 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. > > Signed-off-by: SeongJae Park <sjpark@amazon.de> > --- > drivers/block/xen-blkback/blkback.c | 35 +++++++++++++++++++++++++++-- > 1 file changed, 33 insertions(+), 2 deletions(-) > > diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c > index 3666afa639d1..72d068328ef1 100644 > --- a/drivers/block/xen-blkback/blkback.c > +++ b/drivers/block/xen-blkback/blkback.c > @@ -135,6 +135,31 @@ module_param(log_stats, int, 0644); > /* Number of free pages to remove on each call to gnttab_free_pages */ > #define NUM_BATCH_FREE_PAGES 10 > > +/* > + * Once a memory pressure is detected, keep aggressive shrinking of the free > + * page pools for this time (milliseconds) > + */ > +static int xen_blkif_aggressive_shrinking_duration = 10; > +module_param_named(aggressive_shrinking_duration, > + xen_blkif_aggressive_shrinking_duration, int, 0644); > +MODULE_PARM_DESC(aggressive_shrinking_duration, > +"Duration in ms to do aggressive shrinking when a memory pressure is detected"); > + > +static unsigned long xen_blk_mem_pressure_end; > + > +static unsigned long blkif_shrink_count(struct shrinker *shrinker, > + struct shrink_control *sc) > +{ > + xen_blk_mem_pressure_end = jiffies + > + msecs_to_jiffies(xen_blkif_aggressive_shrinking_duration); > + return 0; > +} > + > +static struct shrinker blkif_shrinker = { > + .count_objects = blkif_shrink_count, > + .seeks = DEFAULT_SEEKS, > +}; > + > static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt) > { > return xen_blkif_pgrant_timeout && > @@ -656,8 +681,11 @@ 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, xen_blk_mem_pressure_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); > @@ -1500,6 +1528,9 @@ static int __init xen_blkif_init(void) > if (rc) > goto failed_init; > > + if (register_shrinker(&blkif_shrinker)) > + pr_warn("shrinker registration failed\n"); > + > failed_init: > return rc; > } CC-ing xen-devel@lists.xenproject.org Thanks, SeongJae Park
diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c index 3666afa639d1..72d068328ef1 100644 --- a/drivers/block/xen-blkback/blkback.c +++ b/drivers/block/xen-blkback/blkback.c @@ -135,6 +135,31 @@ module_param(log_stats, int, 0644); /* Number of free pages to remove on each call to gnttab_free_pages */ #define NUM_BATCH_FREE_PAGES 10 +/* + * Once a memory pressure is detected, keep aggressive shrinking of the free + * page pools for this time (milliseconds) + */ +static int xen_blkif_aggressive_shrinking_duration = 10; +module_param_named(aggressive_shrinking_duration, + xen_blkif_aggressive_shrinking_duration, int, 0644); +MODULE_PARM_DESC(aggressive_shrinking_duration, +"Duration in ms to do aggressive shrinking when a memory pressure is detected"); + +static unsigned long xen_blk_mem_pressure_end; + +static unsigned long blkif_shrink_count(struct shrinker *shrinker, + struct shrink_control *sc) +{ + xen_blk_mem_pressure_end = jiffies + + msecs_to_jiffies(xen_blkif_aggressive_shrinking_duration); + return 0; +} + +static struct shrinker blkif_shrinker = { + .count_objects = blkif_shrink_count, + .seeks = DEFAULT_SEEKS, +}; + static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt) { return xen_blkif_pgrant_timeout && @@ -656,8 +681,11 @@ 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, xen_blk_mem_pressure_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); @@ -1500,6 +1528,9 @@ static int __init xen_blkif_init(void) if (rc) goto failed_init; + if (register_shrinker(&blkif_shrinker)) + pr_warn("shrinker registration failed\n"); + failed_init: return rc; }