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Message-ID: <CAKEwX=ORuJabxQSehU5E0QNG=Gx6QkyTCb1vek3AOcQuvF54Xg@mail.gmail.com>
Date: Tue, 27 Aug 2024 08:23:49 -0700
From: Nhat Pham <nphamcs@...il.com>
To: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
"hannes@...xchg.org" <hannes@...xchg.org>, "yosryahmed@...gle.com" <yosryahmed@...gle.com>,
"ryan.roberts@....com" <ryan.roberts@....com>, "Huang, Ying" <ying.huang@...el.com>,
"21cnbao@...il.com" <21cnbao@...il.com>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"Zou, Nanhai" <nanhai.zou@...el.com>, "Feghali, Wajdi K" <wajdi.k.feghali@...el.com>,
"Gopal, Vinodh" <vinodh.gopal@...el.com>
Subject: Re: [PATCH v4 0/4] mm: ZSWAP swap-out of mTHP folios
On Mon, Aug 26, 2024 at 11:08 PM Sridhar, Kanchana P
<kanchana.p.sridhar@...el.com> wrote:
>
> > Internally, we often see 1-3 or 1-4 saving ratio (or even more).
>
> Agree with this as well. In our experiments with other workloads, we
> typically see much higher ratios.
>
> >
> > Probably does not explain everything, but worth double checking -
> > could you check with zstd to see if the ratio improves.
>
> Sure. I gathered ratio and compressed memory footprint data today with
> 64K mTHP, the 4G SSD swapfile and different zswap compressors.
>
> This patch-series and no zswap charging, 64K mTHP:
> ---------------------------------------------------------------------------
> Total Total Average Average Comp
> compressed compression compressed compression ratio
> length latency length latency
> bytes milliseconds bytes nanoseconds
> ---------------------------------------------------------------------------
> SSD (no zswap) 1,362,296,832 887,861
> lz4 2,610,657,430 55,984 2,055 44,065 1.99
> zstd 729,129,528 50,986 565 39,510 7.25
> deflate-iaa 1,286,533,438 44,785 1,415 49,252 2.89
> ---------------------------------------------------------------------------
>
> zstd does very well on ratio, as expected.
Wait. So zstd is displaying 7-to-1 compression ratio? And has *lower*
average latency?
Why are we running benchmark on lz4 again? Sure there is no free lunch
and no compressor that works well on all kind of data, but lz4's
performance here is so bad that it's borderline justifiable to
disable/bypass zswap with this kind of compresison ratio...
Can I ask you to run benchmarking on zstd from now on?
>
> >
> > >
> > >
> > > Experiment 3 - 4K folios swap characteristics SSD vs. ZSWAP:
> > > ------------------------------------------------------------
> > >
> > > I wanted to take a step back and understand how the mainline v6.11-rc3
> > > handles 4K folios when swapped out to SSD (CONFIG_ZSWAP is off) and
> > when
> > > swapped out to ZSWAP. Interestingly, higher swapout activity is observed
> > > with 4K folios and v6.11-rc3 (with the debug change to not charge zswap to
> > > cgroup).
> > >
> > > v6.11-rc3 with no zswap charge, only 4K folios, no (m)THP:
> > >
> > > -------------------------------------------------------------
> > > SSD (CONFIG_ZSWAP is OFF) ZSWAP lz4 lzo-rle
> > > -------------------------------------------------------------
> > > cgroup memory.events: cgroup memory.events:
> > >
> > > low 0 low 0 0
> > > high 5,068 high 321,923 375,116
> > > max 0 max 0 0
> > > oom 0 oom 0 0
> > > oom_kill 0 oom_kill 0 0
> > > oom_group_kill 0 oom_group_kill 0 0
> > > -------------------------------------------------------------
> > >
> > > SSD (CONFIG_ZSWAP is OFF):
> > > --------------------------
> > > pswpout 415,709
> > > sys time (sec) 301.02
> > > Throughput KB/s 155,970
> > > memcg_high events 5,068
> > > --------------------------
> > >
> > >
> > > ZSWAP lz4 lz4 lz4 lzo-rle
> > > --------------------------------------------------------------
> > > zswpout 1,598,550 1,515,151 1,449,432 1,493,917
> > > sys time (sec) 889.36 481.21 581.22 635.75
> > > Throughput KB/s 35,176 14,765 20,253 21,407
> > > memcg_high events 321,923 412,733 369,976 375,116
> > > --------------------------------------------------------------
> > >
> > > This shows that there is a performance regression of -60% to -195% with
> > > zswap as compared to SSD with 4K folios. The higher swapout activity with
> > > zswap is seen here too (i.e., this doesn't appear to be mTHP-specific).
> > >
> > > I verified this to be the case even with the v6.7 kernel, which also
> > > showed a 2.3X throughput improvement when we don't charge zswap:
> > >
> > > ZSWAP lz4 v6.7 v6.7 with no cgroup zswap charge
> > > --------------------------------------------------------------------
> > > zswpout 1,419,802 1,398,620
> > > sys time (sec) 535.4 613.41
> >
> > systime increases without zswap cgroup charging? That's strange...
>
> Additional data gathered with v6.11-rc3 (listed below) based on your suggestion
> to investigate potential swap.high breaches should hopefully provide some
> explanation.
>
> >
> > > Throughput KB/s 8,671 20,045
> > > memcg_high events 574,046 451,859
> >
> > So, on 4k folio setup, even without cgroup charge, we are still seeing:
> >
> > 1. More zswpout (than observed in SSD)
> > 2. 40-50% worse latency - in fact it is worse without zswap cgroup charging.
> > 3. 100 times the amount of memcg_high events? This is perhaps the
> > *strangest* to me. You're already removing zswap cgroup charging, then
> > where does this comes from? How can we have memory.high violation when
> > zswap does *not* contribute to memory usage?
> >
> > Is this due to swap limit charging? Do you have a cgroup swap limit?
> >
> > mem_high = page_counter_read(&memcg->memory) >
> > READ_ONCE(memcg->memory.high);
> > swap_high = page_counter_read(&memcg->swap) >
> > READ_ONCE(memcg->swap.high);
> > [...]
> >
> > if (mem_high || swap_high) {
> > /*
> > * The allocating tasks in this cgroup will need to do
> > * reclaim or be throttled to prevent further growth
> > * of the memory or swap footprints.
> > *
> > * Target some best-effort fairness between the tasks,
> > * and distribute reclaim work and delay penalties
> > * based on how much each task is actually allocating.
> > */
> > current->memcg_nr_pages_over_high += batch;
> > set_notify_resume(current);
> > break;
> > }
> >
>
> I don't have a swap.high limit set on the cgroup; it is set to "max".
>
> I ran experiments with v6.11-rc3, no zswap charging, 4K folios and different
> zswap compressors to verify if swap.high is breached with the 4G SSD swapfile.
>
> SSD (CONFIG_ZSWAP is OFF):
>
> SSD SSD SSD
> ------------------------------------------------------------
> pswpout 415,709 1,032,170 636,582
> sys time (sec) 301.02 328.15 306.98
> Throughput KB/s 155,970 89,621 122,219
> memcg_high events 5,068 15,072 8,344
> memcg_swap_high events 0 0 0
> memcg_swap_fail events 0 0 0
> ------------------------------------------------------------
>
> ZSWAP zstd zstd zstd
> ----------------------------------------------------------------
> zswpout 1,391,524 1,382,965 1,417,307
> sys time (sec) 474.68 568.24 489.80
> Throughput KB/s 26,099 23,404 111,115
> memcg_high events 335,112 340,335 162,260
> memcg_swap_high events 0 0 0
> memcg_swap_fail events 1,226,899 5,742,153
> (mem_cgroup_try_charge_swap)
> memcg_memory_stat_pgactivate 1,259,547
> (shrink_folio_list)
> ----------------------------------------------------------------
>
> ZSWAP lzo-rle lzo-rle lzo-rle
> -----------------------------------------------------------
> zswpout 1,493,917 1,363,040 1,428,133
> sys time (sec) 635.75 498.63 484.65
> Throughput KB/s 21,407 23,827 20,237
> memcg_high events 375,116 352,814 373,667
> memcg_swap_high events 0 0 0
> memcg_swap_fail events 715,211
> -----------------------------------------------------------
>
> ZSWAP lz4 lz4 lz4 lz4
> ---------------------------------------------------------------------
> zswpout 1,378,781 1,598,550 1,515,151 1,449,432
> sys time (sec) 495.45 889.36 481.21 581.22
> Throughput KB/s 26,248 35,176 14,765 20,253
> memcg_high events 347,209 321,923 412,733 369,976
> memcg_swap_high events 0 0 0 0
> memcg_swap_fail events 580,103 0
> ---------------------------------------------------------------------
>
> ZSWAP deflate-iaa deflate-iaa deflate-iaa
> ----------------------------------------------------------------
> zswpout 380,471 1,440,902 1,397,965
> sys time (sec) 329.06 570.77 467.41
> Throughput KB/s 283,867 28,403 190,600
> memcg_high events 5,551 422,831 28,154
> memcg_swap_high events 0 0 0
> memcg_swap_fail events 0 2,686,758 438,562
> ----------------------------------------------------------------
Why are there 3 columns for each of the compressors? Is this different
runs of the same workload?
And why do some columns have missing cells?
>
> There are no swap.high memcg events recorded in any of the SSD/zswap
> experiments. However, I do see significant number of memcg_swap_fail
> events in some of the zswap runs, for all 3 compressors. This is not
> consistent, because there are some runs with 0 memcg_swap_fail for all
> compressors.
>
> There is a possible co-relation between memcg_swap_fail events
> (/sys/fs/cgroup/test/memory.swap.events) and the high # of memcg_high
> events. The root-cause appears to be that there are no available swap
> slots, memcg_swap_fail is incremented, add_to_swap() fails in
> shrink_folio_list(), followed by "activate_locked:" for the folio.
> The folio re-activation is recorded in cgroup memory.stat pgactivate
> events. The failure to swap out folios due to lack of swap slots could
> contribute towards memory.high breaches.
Yeah FWIW, that was gonna be my first suggestion. This swapfile size
is wayyyy too small...
But that said, the link is not clear to me at all. The only thing I
can think of is lz4's performance sucks so bad that it's not saving
enough memory, leading to regression. And since it's still taking up
swap slot, we cannot use swap either?
>
> However, this is probably not the only cause for either the high # of
> memory.high breaches or the over-reclaim with zswap, as seen in the lz4
> data where the memory.high is significant even in cases where there are no
> memcg_swap_fails.
>
> Some observations/questions based on the above 4K folios swapout data:
>
> 1) There are more memcg_high events as the swapout latency reduces
> (i.e. faster swap-write path). This is even without charging zswap
> utilization to the cgroup.
This is still inexplicable to me. If we are not charging zswap usage,
we shouldn't even be triggering the reclaim_high() path, no?
I'm curious - can you use bpftrace to tracks where/when reclaim_high
is being called?
>
> 2) There appears to be a direct co-relation between higher # of
> memcg_swap_fail events, and an increase in memcg_high breaches and
> reduction in usemem throughput. This combined with the observation in
> (1) suggests that with a faster compressor, we need more swap slots,
> that increases the probability of running out of swap slots with the 4G
> SSD backing device.
>
> 3) Could the data shared earlier on reduction in memcg_high breaches with
> 64K mTHP swapout provide some more clues, if we agree with (1) and (2):
>
> "Interestingly, the # of memcg_high events reduces significantly with 64K
> mTHP as compared to the above 4K memcg_high events data, when tested
> with v4 and no zswap charge: 3,069 (SSD-mTHP) and 19,656 (ZSWAP-mTHP)."
>
> 4) In the case of each zswap compressor, there are some runs that go
> through with 0 memcg_swap_fail events. These runs generally have better
> fewer memcg_high breaches and better sys time/throughput.
>
> 5) For a given swap setup, there is some amount of variance in
> sys time for this workload.
>
> 6) All this suggests that the primary root cause is the concurrency setup,
> where there could be randomness between runs as to the # of processes
> that observe the memory.high breach due to other factors such as
> availability of swap slots for alloc.
>
> To summarize, I believe the root-cause is the 4G SSD swapfile resulting in
> running out of swap slots, and anomalous behavior with over-reclaim when 70
> concurrent processes are working with the 60G memory limit while trying to
> allocate 1G each; with randomness in processes reacting to the breach.
>
> The cgroup zswap charging exacerbates this situation, but is not a problem
> in and of itself.
>
> Nhat, as you pointed out, this is somewhat of an unrealistic scenario that
> doesn't seem to indicate any specific problems to be solved, other than the
> temporary cgroup zswap double-charging.
>
> Would it be fair to evaluate this patch-series based on a more realistic
> swapfile configuration based on 176G ZRAM, for which I had shared the data
> in v2? There weren't any problems with swap slots availability or any
> anomalies that I can think of with this setup, other than the fact that the
> "Before" and "After" sys times could not be directly compared for 2 key
> reasons:
>
> - ZRAM compressed data is not charged to the cgroup, similar to SSD.
> - ZSWAP compressed data is charged to the cgroup.
Yeah that's a bit unfair still. Wild idea, but what about we compare
SSD without zswap (or SSD with zswap, but without this patch series so
that mTHP are not zswapped) v.s zswap-on-zram (i.e with a backing
swapfile on zram block device).
It is stupid, I know. But let's take advantage of the fact that zram
is not charged to cgroup, pretending that its memory foot print is
empty?
I don't know how zram works though, so my apologies if it's a stupid
suggestion :)
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