Re: [patch v2 3/3] mm: page_alloc: fair zone allocator policy
From: Mel Gorman
Date: Thu Aug 08 2013 - 05:21:23 EST
On Thu, Aug 08, 2013 at 12:16:23AM -0400, Johannes Weiner wrote:
> On Wed, Aug 07, 2013 at 11:37:43AM -0400, Johannes Weiner wrote:
> > On Wed, Aug 07, 2013 at 03:58:28PM +0100, Mel Gorman wrote:
> > > On Fri, Aug 02, 2013 at 11:37:26AM -0400, Johannes Weiner wrote:
> > > > @@ -352,6 +352,7 @@ struct zone {
> > > > * free areas of different sizes
> > > > */
> > > > spinlock_t lock;
> > > > + int alloc_batch;
> > > > int all_unreclaimable; /* All pages pinned */
> > > > #if defined CONFIG_COMPACTION || defined CONFIG_CMA
> > > > /* Set to true when the PG_migrate_skip bits should be cleared */
> > >
> > > This adds a dirty cache line that is updated on every allocation even if
> > > it's from the per-cpu allocator. I am concerned that this will introduce
> > > noticable overhead in the allocator paths on large machines running
> > > allocator intensive workloads.
> > >
> > > Would it be possible to move it into the per-cpu pageset? I understand
> > > that hte round-robin nature will then depend on what CPU is running and
> > > the performance characterisics will be different. There might even be an
> > > adverse workload that uses all the batches from all available CPUs until
> > > it is essentially the same problem but that would be a very worst case.
> > > I would hope that in general it would work without adding a big source of
> > > dirty cache line bouncing in the middle of the allocator.
> >
> > Rik made the same suggestion. The per-cpu error is one thing, the
> > problem is if the allocating task and kswapd run on the same CPU and
> > bypass the round-robin allocator completely, at which point we are
> > back to square one. We'd have to reduce the per-cpu lists from a pool
> > to strict batching of frees and allocs without reuse in between. That
> > might be doable, I'll give this another look.
>
> I found a way. It's still in the fast path, but I'm using vmstat
> percpu counters and can stick the update inside the same irq-safe
> section that does the other statistic updates.
>
Ok, there will be some drift as those counters are only updated
periodically or if they overflow. Offhand I think your worst case is
being off by (nr_cpu_in_node * 127 - default_batch) but I doubt it'll be
noticable.
> On a two socket system with a small Normal zone, the results are as
> follows (unfair: mmotm without the fairness allocator, fairpcp: the
> fair allocator + the vmstat optimization):
>
> ---
>
> pft
> mmotm mmotm
> unfair fairpcp
> User 1 0.0258 ( 0.00%) 0.0254 ( 1.40%)
> User 2 0.0264 ( 0.00%) 0.0263 ( 0.21%)
> User 3 0.0271 ( 0.00%) 0.0277 ( -2.36%)
> User 4 0.0287 ( 0.00%) 0.0280 ( 2.33%)
> System 1 0.4904 ( 0.00%) 0.4919 ( -0.29%)
> System 2 0.6141 ( 0.00%) 0.6183 ( -0.68%)
> System 3 0.7346 ( 0.00%) 0.7349 ( -0.04%)
> System 4 0.8700 ( 0.00%) 0.8704 ( -0.05%)
> Elapsed 1 0.5164 ( 0.00%) 0.5182 ( -0.35%)
> Elapsed 2 0.3213 ( 0.00%) 0.3235 ( -0.67%)
> Elapsed 3 0.2800 ( 0.00%) 0.2800 ( 0.00%)
> Elapsed 4 0.2304 ( 0.00%) 0.2303 ( 0.01%)
> Faults/cpu 1 392724.3239 ( 0.00%) 391558.5131 ( -0.30%)
> Faults/cpu 2 319357.5074 ( 0.00%) 317577.8745 ( -0.56%)
> Faults/cpu 3 265703.1420 ( 0.00%) 265668.3579 ( -0.01%)
> Faults/cpu 4 225516.0058 ( 0.00%) 225474.1508 ( -0.02%)
> Faults/sec 1 392051.3043 ( 0.00%) 390880.8201 ( -0.30%)
> Faults/sec 2 635360.7045 ( 0.00%) 631819.1117 ( -0.56%)
> Faults/sec 3 725535.2889 ( 0.00%) 725525.1280 ( -0.00%)
> Faults/sec 4 883307.5047 ( 0.00%) 884026.1721 ( 0.08%)
>
> The overhead appears to be negligible, if not noise.
>
Certainly small enough to not care considering what you're balancing
it against. Mind you, I do note that 4 clients is almost certainly not
enought to load a 2-socket machine. As the test is not memory intensive I
suspect that this test ran entirely within a single socket that would have
avoided the worst costs of dirty cache line bouncing anyway. As the number
of clients grow I predict that the results will become more variable as
it'll depend on scheduling.
> mmotm mmotm
> unfair fairpcp
> User 39.90 39.70
> System 1070.93 1069.50
> Elapsed 557.47 556.86
>
And there is nothing suspicious in the system CPU time.
> <SNIP
>
> parallelio
> mmotm mmotm
> unfair fairpcp
> Ops memcachetest-0M 28012.00 ( 0.00%) 27887.00 ( -0.45%)
> Ops memcachetest-1877M 22366.00 ( 0.00%) 27878.00 ( 24.64%)
> Ops memcachetest-6257M 17770.00 ( 0.00%) 27610.00 ( 55.37%)
> Ops memcachetest-10638M 17695.00 ( 0.00%) 27350.00 ( 54.56%)
> Ops io-duration-0M 0.00 ( 0.00%) 0.00 ( 0.00%)
> Ops io-duration-1877M 42.00 ( 0.00%) 18.00 ( 57.14%)
> Ops io-duration-6257M 97.00 ( 0.00%) 57.00 ( 41.24%)
> Ops io-duration-10638M 172.00 ( 0.00%) 122.00 ( 29.07%)
> Ops swaptotal-0M 0.00 ( 0.00%) 0.00 ( 0.00%)
> Ops swaptotal-1877M 93603.00 ( 0.00%) 0.00 ( 0.00%)
> Ops swaptotal-6257M 113986.00 ( 0.00%) 0.00 ( 0.00%)
> Ops swaptotal-10638M 178887.00 ( 0.00%) 0.00 ( 0.00%)
> Ops swapin-0M 0.00 ( 0.00%) 0.00 ( 0.00%)
> Ops swapin-1877M 20710.00 ( 0.00%) 0.00 ( 0.00%)
> Ops swapin-6257M 18803.00 ( 0.00%) 0.00 ( 0.00%)
> Ops swapin-10638M 18755.00 ( 0.00%) 0.00 ( 0.00%)
> Ops minorfaults-0M 866454.00 ( 0.00%) 844880.00 ( 2.49%)
> Ops minorfaults-1877M 957107.00 ( 0.00%) 845839.00 ( 11.63%)
> Ops minorfaults-6257M 971144.00 ( 0.00%) 844778.00 ( 13.01%)
> Ops minorfaults-10638M 1066811.00 ( 0.00%) 843628.00 ( 20.92%)
> Ops majorfaults-0M 17.00 ( 0.00%) 0.00 ( 0.00%)
> Ops majorfaults-1877M 7636.00 ( 0.00%) 37.00 ( 99.52%)
> Ops majorfaults-6257M 6487.00 ( 0.00%) 37.00 ( 99.43%)
> Ops majorfaults-10638M 7337.00 ( 0.00%) 37.00 ( 99.50%)
>
> Mmtests reporting seems to have a bug calculating the percentage when
> the numbers drop to 0, see swap activity. Those should all be 100%.
>
Yep. I added a note to the TODO list which is now going sideways on the
sheet. Really need to knock a few items off it.
Still, they key takeaway is that it's no longer swapping and with swapins
in particular that is a big deal. I think it's interesting to note that
major faults are also very different, higher than what can be accounted
for by just the swapping. I wonder if the shared libraries and executable
binaries also getting thrown out and paged back in.
> mmotm mmotm
> unfair fairpcp
> User 592.67 695.15
> System 3130.44 3628.81
> Elapsed 7209.01 7206.46
>
I'm not concerned about the high user time here because it's due to not
swapping (I guess). The higher system CPU is interesting and while I expect
it's just because the process is busy I must find the time to see *where*
that time is spent for this workload some day.
> mmotm mmotm
> unfair fairpcp
> Page Ins 1401120 42656
> Page Outs 163980516 153864256
> Swap Ins 316033 0
> Swap Outs 2528278 0
> Alloc DMA 0 0
> Alloc DMA32 59139091 51707843
> Alloc Normal 10013244 16310697
> Alloc Movable 0 0
> Direct pages scanned 210080 235649
> Kswapd pages scanned 61960450 50130023
> Kswapd pages reclaimed 34998767 35769908
> Direct pages reclaimed 179655 173478
> Kswapd efficiency 56% 71%
> Kswapd velocity 8594.863 6956.262
> Direct efficiency 85% 73%
> Direct velocity 29.141 32.700
> Percentage direct scans 0% 0%
> Page writes by reclaim 3523501 1
> Page writes file 995223 1
> Page writes anon 2528278 0
> Page reclaim immediate 2195 9188
> Page rescued immediate 0 0
> Slabs scanned 2048 1536
> Direct inode steals 0 0
> Kswapd inode steals 0 0
> Kswapd skipped wait 0 0
> THP fault alloc 3 3
> THP collapse alloc 4958 3026
> THP splits 28 27
> THP fault fallback 0 0
> THP collapse fail 7 72
> Compaction stalls 65 32
> Compaction success 57 14
> Compaction failures 8 18
> Page migrate success 39460 9899
> Page migrate failure 0 0
> Compaction pages isolated 87140 22017
> Compaction migrate scanned 53494 12526
> Compaction free scanned 913691 396861
> Compaction cost 42 10
> NUMA PTE updates 0 0
> NUMA hint faults 0 0
> NUMA hint local faults 0 0
> NUMA pages migrated 0 0
> AutoNUMA cost 0 0
>
btw, mmtests has another bug and that "Page Ins" and "Page Outs" figures
are completely bogus. They are replaced with simply Major and Minor
faults in my current git tree. Otherwise, note the decrease in kswapd
velocity. That's nice but nowhere near as nice as the elimination of page
writes from reclaim context!
> ---
>
> Patch on top of mmotm:
>
> ---
> From: Johannes Weiner <hannes@xxxxxxxxxxx>
> Subject: [patch] mm: page_alloc: use vmstats for fair zone allocation batching
>
> Avoid dirtying the same cache line with every single page allocation
> by making the fair per-zone allocation batch a vmstat item, which will
> turn it into batched percpu counters on SMP.
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
With this patch on top;
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Thanks very much Johannes, this is a great result. One note below
> @@ -2418,8 +2417,10 @@ static void prepare_slowpath(gfp_t gfp_mask, unsigned int order,
> */
> if (zone_reclaim_mode && !zone_local(preferred_zone, zone))
> continue;
> - zone->alloc_batch = high_wmark_pages(zone) -
> - low_wmark_pages(zone);
> + mod_zone_page_state(zone, NR_ALLOC_BATCH,
> + high_wmark_pages(zone) -
> + low_wmark_pages(zone) -
> + zone_page_state(zone, NR_ALLOC_BATCH));
> }
> }
>
Because of drift this is not exactly equivalent. The batch value will not
be reinitialised but instead adjusted by the delta but it's close enough
to not matter. Due to the size of the delta it will always spill over and
update the global counter so while the decrements from the alloc fast
paths are light, the reset is not. I see no way around this but if there
is still some noticable overhead then I bet this is where it is. It's
still a *LOT* better than what was there before and I think the cost is
justified this time.
--
Mel Gorman
SUSE Labs
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