Re: [PATCH 01/10] mm, page_alloc: Delete the zonelist_cache
From: Vlastimil Babka
Date: Thu Aug 20 2015 - 09:31:03 EST
On 08/12/2015 12:45 PM, Mel Gorman wrote:
The zonelist cache (zlc) was introduced to skip over zones that were
recently known to be full. This avoided expensive operations such as the
cpuset checks, watermark calculations and zone_reclaim. The situation
today is different and the complexity of zlc is harder to justify.
1) The cpuset checks are no-ops unless a cpuset is active and in general are
a lot cheaper.
2) zone_reclaim is now disabled by default and I suspect that was a large
source of the cost that zlc wanted to avoid. When it is enabled, it's
known to be a major source of stalling when nodes fill up and it's
unwise to hit every other user with the overhead.
3) Watermark checks are expensive to calculate for high-order
allocation requests. Later patches in this series will reduce the cost
of the watermark checking.
4) The most important issue is that in the current implementation it
is possible for a failed THP allocation to mark a zone full for order-0
allocations and cause a fallback to remote nodes.
The last issue could be addressed with additional complexity but as the
benefit of zlc is questionable, it is better to remove it. If stalls
due to zone_reclaim are ever reported then an alternative would be to
introduce deferring logic based on a timeout inside zone_reclaim itself
and leave the page allocator fast paths alone.
The impact on page-allocator microbenchmarks is negligible as they don't
hit the paths where the zlc comes into play. The impact was noticeable
in a workload called "stutter". One part uses a lot of anonymous memory,
a second measures mmap latency and a third copies a large file. In an
ideal world the latency application would not notice the mmap latency.
On a 4-node machine the results of this patch are
4-node machine stutter
4.2.0-rc1 4.2.0-rc1
vanilla nozlc-v1r20
Min mmap 53.9902 ( 0.00%) 49.3629 ( 8.57%)
1st-qrtle mmap 54.6776 ( 0.00%) 54.1201 ( 1.02%)
2nd-qrtle mmap 54.9242 ( 0.00%) 54.5961 ( 0.60%)
3rd-qrtle mmap 55.1817 ( 0.00%) 54.9338 ( 0.45%)
Max-90% mmap 55.3952 ( 0.00%) 55.3929 ( 0.00%)
Max-93% mmap 55.4766 ( 0.00%) 57.5712 ( -3.78%)
Max-95% mmap 55.5522 ( 0.00%) 57.8376 ( -4.11%)
Max-99% mmap 55.7938 ( 0.00%) 63.6180 (-14.02%)
Max mmap 6344.0292 ( 0.00%) 67.2477 ( 98.94%)
Mean mmap 57.3732 ( 0.00%) 54.5680 ( 4.89%)
Note the maximum stall latency which was 6 seconds and becomes 67ms with
this patch applied. However, also note that it is not guaranteed this
benchmark always hits pathelogical cases and the milage varies. There is
a secondary impact with more direct reclaim because zones are now being
considered instead of being skipped by zlc.
4.1.0 4.1.0
vanilla nozlc-v1r4
Swap Ins 838 502
Swap Outs 1149395 2622895
DMA32 allocs 17839113 15863747
Normal allocs 129045707 137847920
Direct pages scanned 4070089 29046893
Kswapd pages scanned 17147837 17140694
Kswapd pages reclaimed 17146691 17139601
Direct pages reclaimed 1888879 4886630
Kswapd efficiency 99% 99%
Kswapd velocity 17523.721 17518.928
Direct efficiency 46% 16%
Direct velocity 4159.306 29687.854
Percentage direct scans 19% 62%
Page writes by reclaim 1149395.000 2622895.000
Page writes file 0 0
Page writes anon 1149395 2622895
Interesting, kswapd has no decrease that would counter the increase in
direct reclaim. So there's more reclaim overall. Does it mean that
stutter doesn't like LRU and zlc was disrupting LRU?
The direct page scan and reclaim rates are noticeable. It is possible
this will not be a universal win on all workloads but cycling through
zonelists waiting for zlc->last_full_zap to expire is not the right
decision.
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
It doesn't seem that removal of zlc would increase overhead due to
"expensive operations no longer being avoided". Making some corner-case
benchmark(s) worse as a side-effect of different LRU approximation
shouldn't be a show-stopper. Hence
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
just git grep found some lines that should be also deleted:
include/linux/mmzone.h: * If zlcache_ptr is not NULL, then it is just
the address of zlcache,
include/linux/mmzone.h: * as explained above. If zlcache_ptr is NULL,
there is no zlcache.
And:
@@ -3157,7 +2967,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
struct alloc_context ac = {
.high_zoneidx = gfp_zone(gfp_mask),
- .nodemask = nodemask,
+ .nodemask = nodemask ? : &cpuset_current_mems_allowed,
.migratetype = gfpflags_to_migratetype(gfp_mask),
};
@@ -3188,8 +2998,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
ac.zonelist = zonelist;
/* The preferred zone is used for statistics later */
preferred_zoneref = first_zones_zonelist(ac.zonelist, ac.high_zoneidx,
- ac.nodemask ? : &cpuset_current_mems_allowed,
- &ac.preferred_zone);
+ ac.nodemask, &ac.preferred_zone);
if (!ac.preferred_zone)
goto out;
ac.classzone_idx = zonelist_zone_idx(preferred_zoneref);
These hunks appear unrelated to zonelist cache? Also they move the
evaluation of cpuset_current_mems_allowed
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