| Message ID | 20230526173955.781115-1-cerasuolodomenico@gmail.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] mm: zswap: shrink until can accept | expand |
On Fri, May 26, 2023 at 07:39:55PM +0200, Domenico Cerasuolo wrote: > This update addresses an issue with the zswap reclaim mechanism, which > hinders the efficient offloading of cold pages to disk, thereby > compromising the preservation of the LRU order and consequently > diminishing, if not inverting, its performance benefits. > > The functioning of the zswap shrink worker was found to be inadequate, > as shown by basic benchmark test. For the test, a kernel build was > utilized as a reference, with its memory confined to 1G via a cgroup and > a 5G swap file provided. The results are presented below, these are > averages of three runs without the use of zswap: > > real 46m26s > user 35m4s > sys 7m37s > > With zswap (zbud) enabled and max_pool_percent set to 1 (in a 32G > system), the results changed to: > > real 56m4s > user 35m13s > sys 8m43s > > written_back_pages: 18 > reject_reclaim_fail: 0 > pool_limit_hit:1478 > > Besides the evident regression, one thing to notice from this data is > the extremely low number of written_back_pages and pool_limit_hit. > > The pool_limit_hit counter, which is increased in zswap_frontswap_store > when zswap is completely full, doesn't account for a particular > scenario: once zswap hits his limit, zswap_pool_reached_full is set to > true; with this flag on, zswap_frontswap_store rejects pages if zswap is > still above the acceptance threshold. Once we include the rejections due > to zswap_pool_reached_full && !zswap_can_accept(), the number goes from > 1478 to a significant 21578266. > > Zswap is stuck in an undesirable state where it rejects pages because > it's above the acceptance threshold, yet fails to attempt memory > reclaimation. This happens because the shrink work is only queued when > zswap_frontswap_store detects that it's full and the work itself only > reclaims one page per run. > > This state results in hot pages getting written directly to disk, > while cold ones remain memory, waiting only to be invalidated. The LRU > order is completely broken and zswap ends up being just an overhead > without providing any benefits. > > This commit applies 2 changes: a) the shrink worker is set to reclaim > pages until the acceptance threshold is met and b) the task is also > enqueued when zswap is not full but still above the threshold. > > Testing this suggested update showed much better numbers: > > real 36m37s > user 35m8s > sys 9m32s > > written_back_pages: 10459423 > reject_reclaim_fail: 12896 > pool_limit_hit: 75653 > > V2: > - loop against == -EAGAIN rather than != -EINVAL and also break the loop > on MAX_RECLAIM_RETRIES (thanks Yosry) > - cond_resched() to ensure that the loop doesn't burn the cpu (thanks > Vitaly) > > Fixes: 45190f01dd40 ("mm/zswap.c: add allocation hysteresis if pool limit is hit") > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com> > --- > mm/zswap.c | 15 ++++++++++++--- > 1 file changed, 12 insertions(+), 3 deletions(-) > > diff --git a/mm/zswap.c b/mm/zswap.c > index 59da2a415fbb..f953dceaab34 100644 > --- a/mm/zswap.c > +++ b/mm/zswap.c > @@ -37,6 +37,7 @@ > #include <linux/workqueue.h> > > #include "swap.h" > +#include "internal.h" > > /********************************* > * statistics > @@ -587,9 +588,17 @@ static void shrink_worker(struct work_struct *w) > { > struct zswap_pool *pool = container_of(w, typeof(*pool), > shrink_work); > + int ret, failures = 0; > > - if (zpool_shrink(pool->zpool, 1, NULL)) > - zswap_reject_reclaim_fail++; > + do { > + ret = zpool_shrink(pool->zpool, 1, NULL); > + if (ret) { > + zswap_reject_reclaim_fail++; > + failures++; > + } > + cond_resched(); > + } while (!zswap_can_accept() && ret == -EAGAIN && > + failures < MAX_RECLAIM_RETRIES); It should also loop on !ret, right? AFAIU Yosry's suggestion was that instead of breaking only on -EINVAL, it should break on all failures but -EAGAIN. But it should still keep going if the shrink was successful and the pool cannot accept yet. Basically, something like this? do { ret = zpool_shrink(pool->zpool, 1, NULL); if (ret) { zswap_reject_reclaim_fail++; if (ret != -EAGAIN) break; if (++failures == MAX_RECLAIM_RETRIES) break; } cond_resched(); } while (!zswap_can_accept());
On Fri, May 26, 2023 at 11:10 AM Johannes Weiner <hannes@cmpxchg.org> wrote: > > On Fri, May 26, 2023 at 07:39:55PM +0200, Domenico Cerasuolo wrote: > > This update addresses an issue with the zswap reclaim mechanism, which > > hinders the efficient offloading of cold pages to disk, thereby > > compromising the preservation of the LRU order and consequently > > diminishing, if not inverting, its performance benefits. > > > > The functioning of the zswap shrink worker was found to be inadequate, > > as shown by basic benchmark test. For the test, a kernel build was > > utilized as a reference, with its memory confined to 1G via a cgroup and > > a 5G swap file provided. The results are presented below, these are > > averages of three runs without the use of zswap: > > > > real 46m26s > > user 35m4s > > sys 7m37s > > > > With zswap (zbud) enabled and max_pool_percent set to 1 (in a 32G > > system), the results changed to: > > > > real 56m4s > > user 35m13s > > sys 8m43s > > > > written_back_pages: 18 > > reject_reclaim_fail: 0 > > pool_limit_hit:1478 > > > > Besides the evident regression, one thing to notice from this data is > > the extremely low number of written_back_pages and pool_limit_hit. > > > > The pool_limit_hit counter, which is increased in zswap_frontswap_store > > when zswap is completely full, doesn't account for a particular > > scenario: once zswap hits his limit, zswap_pool_reached_full is set to > > true; with this flag on, zswap_frontswap_store rejects pages if zswap is > > still above the acceptance threshold. Once we include the rejections due > > to zswap_pool_reached_full && !zswap_can_accept(), the number goes from > > 1478 to a significant 21578266. > > > > Zswap is stuck in an undesirable state where it rejects pages because > > it's above the acceptance threshold, yet fails to attempt memory > > reclaimation. This happens because the shrink work is only queued when > > zswap_frontswap_store detects that it's full and the work itself only > > reclaims one page per run. > > > > This state results in hot pages getting written directly to disk, > > while cold ones remain memory, waiting only to be invalidated. The LRU > > order is completely broken and zswap ends up being just an overhead > > without providing any benefits. > > > > This commit applies 2 changes: a) the shrink worker is set to reclaim > > pages until the acceptance threshold is met and b) the task is also > > enqueued when zswap is not full but still above the threshold. > > > > Testing this suggested update showed much better numbers: > > > > real 36m37s > > user 35m8s > > sys 9m32s > > > > written_back_pages: 10459423 > > reject_reclaim_fail: 12896 > > pool_limit_hit: 75653 > > > > V2: > > - loop against == -EAGAIN rather than != -EINVAL and also break the loop > > on MAX_RECLAIM_RETRIES (thanks Yosry) > > - cond_resched() to ensure that the loop doesn't burn the cpu (thanks > > Vitaly) > > > > Fixes: 45190f01dd40 ("mm/zswap.c: add allocation hysteresis if pool limit is hit") > > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com> > > --- > > mm/zswap.c | 15 ++++++++++++--- > > 1 file changed, 12 insertions(+), 3 deletions(-) > > > > diff --git a/mm/zswap.c b/mm/zswap.c > > index 59da2a415fbb..f953dceaab34 100644 > > --- a/mm/zswap.c > > +++ b/mm/zswap.c > > @@ -37,6 +37,7 @@ > > #include <linux/workqueue.h> > > > > #include "swap.h" > > +#include "internal.h" > > > > /********************************* > > * statistics > > @@ -587,9 +588,17 @@ static void shrink_worker(struct work_struct *w) > > { > > struct zswap_pool *pool = container_of(w, typeof(*pool), > > shrink_work); > > + int ret, failures = 0; > > > > - if (zpool_shrink(pool->zpool, 1, NULL)) > > - zswap_reject_reclaim_fail++; > > + do { > > + ret = zpool_shrink(pool->zpool, 1, NULL); > > + if (ret) { > > + zswap_reject_reclaim_fail++; > > + failures++; > > + } > > + cond_resched(); > > + } while (!zswap_can_accept() && ret == -EAGAIN && > > + failures < MAX_RECLAIM_RETRIES); > > It should also loop on !ret, right? > > AFAIU Yosry's suggestion was that instead of breaking only on -EINVAL, > it should break on all failures but -EAGAIN. But it should still keep > going if the shrink was successful and the pool cannot accept yet. > > Basically, something like this? > > do { > ret = zpool_shrink(pool->zpool, 1, NULL); > if (ret) { > zswap_reject_reclaim_fail++; > if (ret != -EAGAIN) > break; > if (++failures == MAX_RECLAIM_RETRIES) > break; > } > cond_resched(); > } while (!zswap_can_accept()); Yes, that's what I meant. Otherwise if shrink is successful we end up doing 1 page only, which is exactly what we are trying to avoid here.
On Fri, May 26, 2023 at 8:10 PM Johannes Weiner <hannes@cmpxchg.org> wrote: > > On Fri, May 26, 2023 at 07:39:55PM +0200, Domenico Cerasuolo wrote: > > This update addresses an issue with the zswap reclaim mechanism, which > > hinders the efficient offloading of cold pages to disk, thereby > > compromising the preservation of the LRU order and consequently > > diminishing, if not inverting, its performance benefits. > > > > The functioning of the zswap shrink worker was found to be inadequate, > > as shown by basic benchmark test. For the test, a kernel build was > > utilized as a reference, with its memory confined to 1G via a cgroup and > > a 5G swap file provided. The results are presented below, these are > > averages of three runs without the use of zswap: > > > > real 46m26s > > user 35m4s > > sys 7m37s > > > > With zswap (zbud) enabled and max_pool_percent set to 1 (in a 32G > > system), the results changed to: > > > > real 56m4s > > user 35m13s > > sys 8m43s > > > > written_back_pages: 18 > > reject_reclaim_fail: 0 > > pool_limit_hit:1478 > > > > Besides the evident regression, one thing to notice from this data is > > the extremely low number of written_back_pages and pool_limit_hit. > > > > The pool_limit_hit counter, which is increased in zswap_frontswap_store > > when zswap is completely full, doesn't account for a particular > > scenario: once zswap hits his limit, zswap_pool_reached_full is set to > > true; with this flag on, zswap_frontswap_store rejects pages if zswap is > > still above the acceptance threshold. Once we include the rejections due > > to zswap_pool_reached_full && !zswap_can_accept(), the number goes from > > 1478 to a significant 21578266. > > > > Zswap is stuck in an undesirable state where it rejects pages because > > it's above the acceptance threshold, yet fails to attempt memory > > reclaimation. This happens because the shrink work is only queued when > > zswap_frontswap_store detects that it's full and the work itself only > > reclaims one page per run. > > > > This state results in hot pages getting written directly to disk, > > while cold ones remain memory, waiting only to be invalidated. The LRU > > order is completely broken and zswap ends up being just an overhead > > without providing any benefits. > > > > This commit applies 2 changes: a) the shrink worker is set to reclaim > > pages until the acceptance threshold is met and b) the task is also > > enqueued when zswap is not full but still above the threshold. > > > > Testing this suggested update showed much better numbers: > > > > real 36m37s > > user 35m8s > > sys 9m32s > > > > written_back_pages: 10459423 > > reject_reclaim_fail: 12896 > > pool_limit_hit: 75653 > > > > V2: > > - loop against == -EAGAIN rather than != -EINVAL and also break the loop > > on MAX_RECLAIM_RETRIES (thanks Yosry) > > - cond_resched() to ensure that the loop doesn't burn the cpu (thanks > > Vitaly) > > > > Fixes: 45190f01dd40 ("mm/zswap.c: add allocation hysteresis if pool limit is hit") > > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com> > > --- > > mm/zswap.c | 15 ++++++++++++--- > > 1 file changed, 12 insertions(+), 3 deletions(-) > > > > diff --git a/mm/zswap.c b/mm/zswap.c > > index 59da2a415fbb..f953dceaab34 100644 > > --- a/mm/zswap.c > > +++ b/mm/zswap.c > > @@ -37,6 +37,7 @@ > > #include <linux/workqueue.h> > > > > #include "swap.h" > > +#include "internal.h" > > > > /********************************* > > * statistics > > @@ -587,9 +588,17 @@ static void shrink_worker(struct work_struct *w) > > { > > struct zswap_pool *pool = container_of(w, typeof(*pool), > > shrink_work); > > + int ret, failures = 0; > > > > - if (zpool_shrink(pool->zpool, 1, NULL)) > > - zswap_reject_reclaim_fail++; > > + do { > > + ret = zpool_shrink(pool->zpool, 1, NULL); > > + if (ret) { > > + zswap_reject_reclaim_fail++; > > + failures++; > > + } > > + cond_resched(); > > + } while (!zswap_can_accept() && ret == -EAGAIN && > > + failures < MAX_RECLAIM_RETRIES); > > It should also loop on !ret, right? > > AFAIU Yosry's suggestion was that instead of breaking only on -EINVAL, > it should break on all failures but -EAGAIN. But it should still keep > going if the shrink was successful and the pool cannot accept yet. > > Basically, something like this? > > do { > ret = zpool_shrink(pool->zpool, 1, NULL); > if (ret) { > zswap_reject_reclaim_fail++; > if (ret != -EAGAIN) > break; > if (++failures == MAX_RECLAIM_RETRIES) > break; > } > cond_resched(); > } while (!zswap_can_accept()); Thanks, !ret should indeed keep it going.
diff --git a/mm/zswap.c b/mm/zswap.c index 59da2a415fbb..f953dceaab34 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -37,6 +37,7 @@ #include <linux/workqueue.h> #include "swap.h" +#include "internal.h" /********************************* * statistics @@ -587,9 +588,17 @@ static void shrink_worker(struct work_struct *w) { struct zswap_pool *pool = container_of(w, typeof(*pool), shrink_work); + int ret, failures = 0; - if (zpool_shrink(pool->zpool, 1, NULL)) - zswap_reject_reclaim_fail++; + do { + ret = zpool_shrink(pool->zpool, 1, NULL); + if (ret) { + zswap_reject_reclaim_fail++; + failures++; + } + cond_resched(); + } while (!zswap_can_accept() && ret == -EAGAIN && + failures < MAX_RECLAIM_RETRIES); zswap_pool_put(pool); } @@ -1188,7 +1197,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset, if (zswap_pool_reached_full) { if (!zswap_can_accept()) { ret = -ENOMEM; - goto reject; + goto shrink; } else zswap_pool_reached_full = false; }
This update addresses an issue with the zswap reclaim mechanism, which hinders the efficient offloading of cold pages to disk, thereby compromising the preservation of the LRU order and consequently diminishing, if not inverting, its performance benefits. The functioning of the zswap shrink worker was found to be inadequate, as shown by basic benchmark test. For the test, a kernel build was utilized as a reference, with its memory confined to 1G via a cgroup and a 5G swap file provided. The results are presented below, these are averages of three runs without the use of zswap: real 46m26s user 35m4s sys 7m37s With zswap (zbud) enabled and max_pool_percent set to 1 (in a 32G system), the results changed to: real 56m4s user 35m13s sys 8m43s written_back_pages: 18 reject_reclaim_fail: 0 pool_limit_hit:1478 Besides the evident regression, one thing to notice from this data is the extremely low number of written_back_pages and pool_limit_hit. The pool_limit_hit counter, which is increased in zswap_frontswap_store when zswap is completely full, doesn't account for a particular scenario: once zswap hits his limit, zswap_pool_reached_full is set to true; with this flag on, zswap_frontswap_store rejects pages if zswap is still above the acceptance threshold. Once we include the rejections due to zswap_pool_reached_full && !zswap_can_accept(), the number goes from 1478 to a significant 21578266. Zswap is stuck in an undesirable state where it rejects pages because it's above the acceptance threshold, yet fails to attempt memory reclaimation. This happens because the shrink work is only queued when zswap_frontswap_store detects that it's full and the work itself only reclaims one page per run. This state results in hot pages getting written directly to disk, while cold ones remain memory, waiting only to be invalidated. The LRU order is completely broken and zswap ends up being just an overhead without providing any benefits. This commit applies 2 changes: a) the shrink worker is set to reclaim pages until the acceptance threshold is met and b) the task is also enqueued when zswap is not full but still above the threshold. Testing this suggested update showed much better numbers: real 36m37s user 35m8s sys 9m32s written_back_pages: 10459423 reject_reclaim_fail: 12896 pool_limit_hit: 75653 V2: - loop against == -EAGAIN rather than != -EINVAL and also break the loop on MAX_RECLAIM_RETRIES (thanks Yosry) - cond_resched() to ensure that the loop doesn't burn the cpu (thanks Vitaly) Fixes: 45190f01dd40 ("mm/zswap.c: add allocation hysteresis if pool limit is hit") Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com> --- mm/zswap.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-)