Re: [PATCH] vmscan: retry without cache trim mode if nothing scanned
Hello, On Sun, Mar 14, 2021 at 01:58:33PM -0700, Shakeel Butt wrote: > > If my understanding were correct, what Tejun suggested is to add a fast > > read interface to rstat to be used in hot path. And its accuracy is > > similar as that of traditional per-CPU counter. But if we can regularly > > update the lruvec rstat with something like vmstat_update(), that should > > be OK for the issue described in this patch. > > > > This is also my understanding. Tejun, please correct us if we misunderstood > you. Yeah, that was what I had on mind. Instead of always flushing on read, have a variant where flushing is explicit and trigger that periodically (or whichever way appropriate). Thanks. -- tejun
Re: [PATCH] vmscan: retry without cache trim mode if nothing scanned
On Thu, Mar 11, 2021 at 12:52 AM Huang, Ying wrote: > > Hi, Butt, > > Shakeel Butt writes: > > > On Wed, Mar 10, 2021 at 4:47 PM Huang, Ying wrote: > >> > >> From: Huang Ying > >> > >> In shrink_node(), to determine whether to enable cache trim mode, the > >> LRU size is gotten via lruvec_page_state(). That gets the value from > >> a per-CPU counter (mem_cgroup_per_node->lruvec_stat[]). The error of > >> the per-CPU counter from CPU local counting and the descendant memory > >> cgroups may cause some issues. We run into this in 0-Day performance > >> test. > >> > >> 0-Day uses the RAM file system as root file system, so the number of > >> the reclaimable file pages is very small. In the swap testing, the > >> inactive file LRU list will become almost empty soon. But the size of > >> the inactive file LRU list gotten from the per-CPU counter may keep a > >> much larger value (say, 33, 50, etc.). This will enable cache trim > >> mode, but nothing can be scanned in fact. The following pattern > >> repeats for long time in the test, > >> > >> priorityinactive_file_size cache_trim_mode > >> 12 33 0 > >> 11 33 0 > >> ... > >> 6 33 0 > >> 5 33 1 > >> ... > >> 1 33 1 > >> > >> That is, the cache_trim_mode will be enabled wrongly when the scan > >> priority decreases to 5. And the problem will not be recovered for > >> long time. > >> > >> It's hard to get the more accurate size of the inactive file list > >> without much more overhead. And it's hard to estimate the error of > >> the per-CPU counter too, because there may be many descendant memory > >> cgroups. But after the actual scanning, if nothing can be scanned > >> with the cache trim mode, it should be wrong to enable the cache trim > >> mode. So we can retry with the cache trim mode disabled. This patch > >> implement this policy. > > > > Instead of playing with the already complicated heuristics, we should > > improve the accuracy of the lruvec stats. Johannes already fixed the > > memcg stats using rstat infrastructure and Tejun has suggestions on > > how to use rstat infrastructure efficiently for lruvec stats at > > https://lore.kernel.org/linux-mm/ycfgr300eriez...@slm.duckdns.org/. > > Thanks for your information! It should be better if we can improve the > accuracy of lruvec stats without much overhead. But that may be not a > easy task. > > If my understanding were correct, what Tejun suggested is to add a fast > read interface to rstat to be used in hot path. And its accuracy is > similar as that of traditional per-CPU counter. But if we can regularly > update the lruvec rstat with something like vmstat_update(), that should > be OK for the issue described in this patch. > This is also my understanding. Tejun, please correct us if we misunderstood you. BTW Johannes was working on rstat-based lruvec stats patch. Johannes, are you planning to work on the optimization Tejun has suggested.
Re: [PATCH] vmscan: retry without cache trim mode if nothing scanned
Hi, Butt, Shakeel Butt writes: > On Wed, Mar 10, 2021 at 4:47 PM Huang, Ying wrote: >> >> From: Huang Ying >> >> In shrink_node(), to determine whether to enable cache trim mode, the >> LRU size is gotten via lruvec_page_state(). That gets the value from >> a per-CPU counter (mem_cgroup_per_node->lruvec_stat[]). The error of >> the per-CPU counter from CPU local counting and the descendant memory >> cgroups may cause some issues. We run into this in 0-Day performance >> test. >> >> 0-Day uses the RAM file system as root file system, so the number of >> the reclaimable file pages is very small. In the swap testing, the >> inactive file LRU list will become almost empty soon. But the size of >> the inactive file LRU list gotten from the per-CPU counter may keep a >> much larger value (say, 33, 50, etc.). This will enable cache trim >> mode, but nothing can be scanned in fact. The following pattern >> repeats for long time in the test, >> >> priorityinactive_file_size cache_trim_mode >> 12 33 0 >> 11 33 0 >> ... >> 6 33 0 >> 5 33 1 >> ... >> 1 33 1 >> >> That is, the cache_trim_mode will be enabled wrongly when the scan >> priority decreases to 5. And the problem will not be recovered for >> long time. >> >> It's hard to get the more accurate size of the inactive file list >> without much more overhead. And it's hard to estimate the error of >> the per-CPU counter too, because there may be many descendant memory >> cgroups. But after the actual scanning, if nothing can be scanned >> with the cache trim mode, it should be wrong to enable the cache trim >> mode. So we can retry with the cache trim mode disabled. This patch >> implement this policy. > > Instead of playing with the already complicated heuristics, we should > improve the accuracy of the lruvec stats. Johannes already fixed the > memcg stats using rstat infrastructure and Tejun has suggestions on > how to use rstat infrastructure efficiently for lruvec stats at > https://lore.kernel.org/linux-mm/ycfgr300eriez...@slm.duckdns.org/. Thanks for your information! It should be better if we can improve the accuracy of lruvec stats without much overhead. But that may be not a easy task. If my understanding were correct, what Tejun suggested is to add a fast read interface to rstat to be used in hot path. And its accuracy is similar as that of traditional per-CPU counter. But if we can regularly update the lruvec rstat with something like vmstat_update(), that should be OK for the issue described in this patch. Best Regards, Huang, Ying
Re: [PATCH] vmscan: retry without cache trim mode if nothing scanned
On Wed, Mar 10, 2021 at 4:47 PM Huang, Ying wrote: > > From: Huang Ying > > In shrink_node(), to determine whether to enable cache trim mode, the > LRU size is gotten via lruvec_page_state(). That gets the value from > a per-CPU counter (mem_cgroup_per_node->lruvec_stat[]). The error of > the per-CPU counter from CPU local counting and the descendant memory > cgroups may cause some issues. We run into this in 0-Day performance > test. > > 0-Day uses the RAM file system as root file system, so the number of > the reclaimable file pages is very small. In the swap testing, the > inactive file LRU list will become almost empty soon. But the size of > the inactive file LRU list gotten from the per-CPU counter may keep a > much larger value (say, 33, 50, etc.). This will enable cache trim > mode, but nothing can be scanned in fact. The following pattern > repeats for long time in the test, > > priorityinactive_file_size cache_trim_mode > 12 33 0 > 11 33 0 > ... > 6 33 0 > 5 33 1 > ... > 1 33 1 > > That is, the cache_trim_mode will be enabled wrongly when the scan > priority decreases to 5. And the problem will not be recovered for > long time. > > It's hard to get the more accurate size of the inactive file list > without much more overhead. And it's hard to estimate the error of > the per-CPU counter too, because there may be many descendant memory > cgroups. But after the actual scanning, if nothing can be scanned > with the cache trim mode, it should be wrong to enable the cache trim > mode. So we can retry with the cache trim mode disabled. This patch > implement this policy. Instead of playing with the already complicated heuristics, we should improve the accuracy of the lruvec stats. Johannes already fixed the memcg stats using rstat infrastructure and Tejun has suggestions on how to use rstat infrastructure efficiently for lruvec stats at https://lore.kernel.org/linux-mm/ycfgr300eriez...@slm.duckdns.org/.
[PATCH] vmscan: retry without cache trim mode if nothing scanned
From: Huang Ying
In shrink_node(), to determine whether to enable cache trim mode, the
LRU size is gotten via lruvec_page_state(). That gets the value from
a per-CPU counter (mem_cgroup_per_node->lruvec_stat[]). The error of
the per-CPU counter from CPU local counting and the descendant memory
cgroups may cause some issues. We run into this in 0-Day performance
test.
0-Day uses the RAM file system as root file system, so the number of
the reclaimable file pages is very small. In the swap testing, the
inactive file LRU list will become almost empty soon. But the size of
the inactive file LRU list gotten from the per-CPU counter may keep a
much larger value (say, 33, 50, etc.). This will enable cache trim
mode, but nothing can be scanned in fact. The following pattern
repeats for long time in the test,
priorityinactive_file_size cache_trim_mode
12 33 0
11 33 0
...
6 33 0
5 33 1
...
1 33 1
That is, the cache_trim_mode will be enabled wrongly when the scan
priority decreases to 5. And the problem will not be recovered for
long time.
It's hard to get the more accurate size of the inactive file list
without much more overhead. And it's hard to estimate the error of
the per-CPU counter too, because there may be many descendant memory
cgroups. But after the actual scanning, if nothing can be scanned
with the cache trim mode, it should be wrong to enable the cache trim
mode. So we can retry with the cache trim mode disabled. This patch
implement this policy.
The test results for pmbench with normal access address distribution
and 2 NVMe disks as swap on a Intel server for the base and patched
kernel are as follows.
base changepatched
-- -- -
12210 18.6% 14480 pmbench.throughput.aps
164078 -56.3% 71686 vmstat.swap.si
166511 -20.8% 131957 vmstat.swap.so
1992 -52.2% 953.17 proc-vmstat.kswapd_high_wmark_hit_quickly
230.20 -63.8% 83.33 proc-vmstat.kswapd_low_wmark_hit_quickly
2318 -41.4% 1358 proc-vmstat.pageoutrun
>From the above table, with the patch, the page reclaiming algorithm
runs more smoothly, the hot/cold pages distinguishing works better, so
the swap in/out throughput decreases considerably, and the benchmark
throughput increases 18.6%.
Signed-off-by: "Huang, Ying"
Cc: Mel Gorman
Cc: Johannes Weiner
Cc: Vladimir Davydov
Cc: Michal Hocko
Cc: Joonsoo Kim
---
mm/vmscan.c | 19 +++
1 file changed, 15 insertions(+), 4 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index fea6b43bc1f9..1304e3c98a14 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2606,7 +2606,8 @@ static inline bool should_continue_reclaim(struct
pglist_data *pgdat,
return inactive_lru_pages > pages_for_compaction;
}
-static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
+static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc,
+ bool skip_slab)
{
struct mem_cgroup *target_memcg = sc->target_mem_cgroup;
struct mem_cgroup *memcg;
@@ -2652,8 +2653,9 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct
scan_control *sc)
shrink_lruvec(lruvec, sc);
- shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,
- sc->priority);
+ if (!skip_slab)
+ shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,
+ sc->priority);
/* Record the group's reclaim efficiency */
vmpressure(sc->gfp_mask, memcg, false,
@@ -2669,6 +2671,7 @@ static void shrink_node(pg_data_t *pgdat, struct
scan_control *sc)
unsigned long nr_reclaimed, nr_scanned;
struct lruvec *target_lruvec;
bool reclaimable = false;
+ bool skip_slab;
unsigned long file;
target_lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup, pgdat);
@@ -2767,7 +2770,15 @@ static void shrink_node(pg_data_t *pgdat, struct
scan_control *sc)
anon >> sc->priority;
}
- shrink_node_memcgs(pgdat, sc);
+ skip_slab = false;
+retry:
+ shrink_node_memcgs(pgdat, sc, skip_slab);
+ /* Nothing can be scanned with cache trim mode, retry without it */
+ if (sc->cache_trim_mode && sc->nr_scanned == nr_scanned) {
+ sc->cache_trim_mode = 0;
+ skip_slab = true;
+ goto retry;
+ }
if (reclaim_state) {
sc->nr_reclaimed += reclaim_state->reclaimed_slab;
--
2.30.1
