Re: [RFC 0/4] mm: zswap: add support for zswapin of large folios

Barry Song
2024-10-22T22:07:13Z

On Wed, Oct 23, 2024 at 10:17 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 22/10/2024 21:46, Barry Song wrote:
> > On Wed, Oct 23, 2024 at 4:26 AM Usama Arif <usamaarif642@gmail.com> wrote:
> >>
> >>
> >>
> >> On 21/10/2024 11:40, Usama Arif wrote:
> >>>
> >>>
> >>> On 21/10/2024 06:09, Barry Song wrote:
> >>>> On Fri, Oct 18, 2024 at 11:50 PM Usama Arif <usamaarif642@gmail.com> wrote:
> >>>>>
> >>>>> After large folio zswapout support added in [1], this patch adds
> >>>>> support for zswapin of large folios to bring it on par with zram.
> >>>>> This series makes sure that the benefits of large folios (fewer
> >>>>> page faults, batched PTE and rmap manipulation, reduced lru list,
> >>>>> TLB coalescing (for arm64 and amd)) are not lost at swap out when
> >>>>> using zswap.
> >>>>>
> >>>>> It builds on top of [2] which added large folio swapin support for
> >>>>> zram and provides the same level of large folio swapin support as
> >>>>> zram, i.e. only supporting swap count == 1.
> >>>>>
> >>>>> Patch 1 skips swapcache for swapping in zswap pages, this should improve
> >>>>> no readahead swapin performance [3], and also allows us to build on large
> >>>>> folio swapin support added in [2], hence is a prerequisite for patch 3.
> >>>>>
> >>>>> Patch 3 adds support for large folio zswapin. This patch does not add
> >>>>> support for hybrid backends (i.e. folios partly present swap and zswap).
> >>>>>
> >>>>> The main performance benefit comes from maintaining large folios *after*
> >>>>> swapin, large folio performance improvements have been mentioned in previous
> >>>>> series posted on it [2],[4], so have not added those. Below is a simple
> >>>>> microbenchmark to measure the time needed *for* zswpin of 1G memory (along
> >>>>> with memory integrity check).
> >>>>>
> >>>>> | no mTHP (ms) | 1M mTHP enabled (ms)
> >>>>> Base kernel | 1165 | 1163
> >>>>> Kernel with mTHP zswpin series | 1203 | 738
> >>>>
> >>>> Hi Usama,
> >>>> Do you know where this minor regression for non-mTHP comes from?
> >>>> As you even have skipped swapcache for small folios in zswap in patch1,
> >>>> that part should have some gain? is it because of zswap_present_test()?
> >>>>
> >>>
> >>> Hi Barry,
> >>>
> >>> The microbenchmark does a sequential read of 1G of memory, so it probably
> >>> isnt very representative of real world usecases. This also means that
> >>> swap_vma_readahead is able to readahead accurately all pages in its window.
> >>> With this patch series, if doing 4K swapin, you get 1G/4K calls of fast
> >>> do_swap_page. Without this patch, you get 1G/(4K*readahead window) of slow
> >>> do_swap_page calls. I had added some prints and I was seeing 8 pages being
> >>> readahead in 1 do_swap_page. The larger number of calls causes the slight
> >>> regression (eventhough they are quite fast). I think in a realistic scenario,
> >>> where readahead window wont be as large, there wont be a regression.
> >>> The cost of zswap_present_test in the whole call stack of swapping page is
> >>> very low and I think can be ignored.
> >>>
> >>> I think the more interesting thing is what Kanchana pointed out in
> >>> https://lore.kernel.org/all/f2f2053f-ec5f-46a4-800d-50a3d2e61bff@gmail.com/
> >>> I am curious, did you see this when testing large folio swapin and compression
> >>> at 4K granuality? Its looks like swap thrashing so I think it would be common
> >>> between zswap and zram. I dont have larger granuality zswap compression done,
> >>> which is why I think there is a regression in time taken. (It could be because
> >>> its tested on intel as well).
> >>>
> >>> Thanks,
> >>> Usama
> >>>
> >>
> >> Hi,
> >>
> >> So I have been doing some benchmarking after Kanchana pointed out a performance
> >> regression in [1] of swapping in large folio. I would love to get thoughts from
> >> zram folks on this, as thats where large folio swapin was first added [2].
> >> As far as I can see, the current support in zram is doing large folio swapin
> >> at 4K granuality. The large granuality compression in [3] which was posted
> >> in March is not merged, so I am currently comparing upstream zram with this series.
> >>
> >> With the microbenchmark below of timing 1G swapin, there was a very large improvement
> >> in performance by using this series. I think similar numbers would be seen in zram.
> >
> > Imagine running several apps on a phone and switching
> > between them: A → B → C → D → E … → A → B … The app
> > currently on the screen retains its memory, while the ones
> > sent to the background are swapped out. When we bring
> > those apps back to the foreground, their memory is restored.
> > This behavior is quite similar to what you're seeing with
> > your microbenchmark.
> >
>
> Hi Barry,
>
> Thanks for explaining this! Do you know if there is some open source benchmark
> we could use to show an improvement in app switching with large folios?
>

I’m fairly certain the Android team has this benchmark, but it’s not
open source.

A straightforward way to simulate this is to use a script that
cyclically launches multiple applications, such as Chrome, Firefox,
Office, PDF, and others.

for example:

launch chrome;
launch firefox;
launch youtube;
....
launch chrome;
launch firefox;
....

On Android, we have "Android activity manager 'am' command" to do that.
https://gist.github.com/tsohr/5711945

Not quite sure if other windows managers have similar tools.

> Also I guess swap thrashing can happen when apps are brought back to foreground?
>

Typically, the foreground app doesn't experience much swapping,
as it is the most recently or frequently used. However, this may
not hold for very low-end phones, where memory is significantly
less than the app's working set. For instance, we can't expect a
good user experience when playing a large game that requires 8GB
of memory on a 4GB phone! :-)
And for low-end phones, we never even enable mTHP.

> >>
> >> But when doing kernel build test, Kanchana saw a regression in [1]. I believe
> >> its because of swap thrashing (causing large zswap activity), due to larger page swapin.
> >> The part of the code that decides large folio swapin is the same between zswap and zram,
> >> so I believe this would be observed in zram as well.
> >
> > Is this an extreme case where the workload's working set far
> > exceeds the available memory by memcg limitation? I doubt mTHP
> > would provide any real benefit from the start if the workload is bound to
> > experience swap thrashing. What if we disable mTHP entirely?
> >
>
> I would agree, this is an extreme case. I wanted (z)swap activity to happen so limited
> memory.max to 4G.
>
> mTHP is beneficial in kernel test benchmarking going from no mTHP to 16K:
>
> ARM make defconfig; time make -j$(nproc) Image, cgroup memory.max=4G
> metric no mTHP 16K mTHP=always
> real 1m0.613s 0m52.008s
> user 25m23.028s 25m19.488s
> sys 25m45.466s 18m11.640s
> zswpin 1911194 3108438
> zswpout 6880815 9374628
> pgfault 120430166 48976658
> pgmajfault 1580674 2327086
>
>

Interesting! We never use a phone to build the Linux kernel, but
let me see if I can find some other machines to reproduce your data.

>
>
> >>
> >> My initial thought was this might be because its intel, where you dont have the advantage
> >> of TLB coalescing, so tested on AMD and ARM, but the regression is there on AMD
> >> and ARM as well, though a bit less (have added the numbers below).
> >>
> >> The numbers show that the zswap activity increases and page faults decrease.
> >> Overall this does result in sys time increasing and real time slightly increases,
> >> likely because the cost of increased zswap activity is more than the benefit of
> >> lower page faults.
> >> I can see in [3] that pagefaults reduced in zram as well.
> >>
> >> Large folio swapin shows good numbers in microbenchmarks that just target reduce page
> >> faults and sequential swapin only, but not in kernel build test. Is a similar regression
> >> observed with zram when enabling large folio swapin on kernel build test? Maybe large
> >> folio swapin makes more sense on workloads where mappings are kept for a longer time?
> >>
> >
> > I suspect this is because mTHP doesn't always benefit workloads
> > when available memory is quite limited compared to the working set.
> > In that case, mTHP swap-in might introduce more features that
> > exacerbate the problem. We used to have an extra control "swapin_enabled"
> > for swap-in, but it never gained much traction:
> > https://lore.kernel.org/linux-mm/20240726094618.401593-5-21cnbao@gmail.com/
> > We can reconsider whether to include the knob, but if it's better
> > to disable mTHP entirely for these cases, we can still adhere to
> > the policy of "enabled".
> >
> Yes I think this makes sense to have. The only thing is, its too many knobs!
> I personally think its already difficult to decide upto which mTHP size we
> should enable (and I think this changes per workload). But if we add swapin_enabled
> on top of that it can make things more difficult.
>
> > Using large block compression and decompression in zRAM will
> > significantly reduce CPU usage, likely making the issue unnoticeable.
> > However, the default minimum size for large block support is currently
> > set to 64KB(ZSMALLOC_MULTI_PAGES_ORDER = 4).
> >
>
> I saw that the patch was sent in March, and there werent any updates after?
> Maybe I can try and cherry-pick that and see if we can develop large
> granularity compression for zswap.

will provide an updated version next week.

>
> >>
> >> Kernel build numbers in cgroup with memory.max=4G to trigger zswap
> >> Command for AMD: make defconfig; time make -j$(nproc) bzImage
> >> Command for ARM: make defconfig; time make -j$(nproc) Image
> >>
> >>
> >> AMD 16K+32K THP=always
> >> metric mm-unstable mm-unstable + large folio zswapin series
> >> real 1m23.038s 1m23.050s
> >> user 53m57.210s 53m53.437s
> >> sys 7m24.592s 7m48.843s
> >> zswpin 612070 999244
> >> zswpout 2226403 2347979
> >> pgfault 20667366 20481728
> >> pgmajfault 385887 269117
> >>
> >> AMD 16K+32K+64K THP=always
> >> metric mm-unstable mm-unstable + large folio zswapin series
> >> real 1m22.975s 1m23.266s
> >> user 53m51.302s 53m51.069s
> >> sys 7m40.168s 7m57.104s
> >> zswpin 676492 1258573
> >> zswpout 2449839 2714767
> >> pgfault 17540746 17296555
> >> pgmajfault 429629 307495
> >> --------------------------
> >> ARM 16K+32K THP=always
> >> metric mm-unstable mm-unstable + large folio zswapin series
> >> real 0m51.168s 0m52.086s
> >> user 25m14.715s 25m15.765s
> >> sys 17m18.856s 18m8.031s
> >> zswpin 3904129 7339245
> >> zswpout 11171295 13473461
> >> pgfault 37313345 36011338
> >> pgmajfault 2726253 1932642
> >>
> >>
> >> ARM 16K+32K+64K THP=always
> >> metric mm-unstable mm-unstable + large folio zswapin series
> >> real 0m52.017s 0m53.828s
> >> user 25m2.742s 25m0.046s
> >> sys 18m24.525s 20m26.207s
> >> zswpin 4853571 8908664
> >> zswpout 12297199 15768764
> >> pgfault 32158152 30425519
> >> pgmajfault 3320717 2237015
> >>
> >>
> >> Thanks!
> >> Usama
> >>
> >>
> >> [1] https://lore.kernel.org/all/f2f2053f-ec5f-46a4-800d-50a3d2e61bff@gmail.com/
> >> [2] https://lore.kernel.org/all/20240821074541.516249-3-hanchuanhua@oppo.com/
> >> [3] https://lore.kernel.org/all/20240327214816.31191-1-21cnbao@gmail.com/
> >>
> >>>
> >>>>>
> >>>>> The time measured was pretty consistent between runs (~1-2% variation).
> >>>>> There is 36% improvement in zswapin time with 1M folios. The percentage
> >>>>> improvement is likely to be more if the memcmp is removed.
> >>>>>
> >>>>> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> >>>>> index 40de679248b8..77068c577c86 100644
> >>>>> --- a/tools/testing/selftests/cgroup/test_zswap.c
> >>>>> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> >>>>> @@ -9,6 +9,8 @@
> >>>>> #include <string.h>
> >>>>> #include <sys/wait.h>
> >>>>> #include <sys/mman.h>
> >>>>> +#include <sys/time.h>
> >>>>> +#include <malloc.h>
> >>>>>
> >>>>> #include "../kselftest.h"
> >>>>> #include "cgroup_util.h"
> >>>>> @@ -407,6 +409,74 @@ static int test_zswap_writeback_disabled(const char *root)
> >>>>> return test_zswap_writeback(root, false);
> >>>>> }
> >>>>>
> >>>>> +static int zswapin_perf(const char *cgroup, void *arg)
> >>>>> +{
> >>>>> + long pagesize = sysconf(_SC_PAGESIZE);
> >>>>> + size_t memsize = MB(1*1024);
> >>>>> + char buf[pagesize];
> >>>>> + int ret = -1;
> >>>>> + char *mem;
> >>>>> + struct timeval start, end;
> >>>>> +
> >>>>> + mem = (char *)memalign(2*1024*1024, memsize);
> >>>>> + if (!mem)
> >>>>> + return ret;
> >>>>> +
> >>>>> + /*
> >>>>> + * Fill half of each page with increasing data, and keep other
> >>>>> + * half empty, this will result in data that is still compressible
> >>>>> + * and ends up in zswap, with material zswap usage.
> >>>>> + */
> >>>>> + for (int i = 0; i < pagesize; i++)
> >>>>> + buf[i] = i < pagesize/2 ? (char) i : 0;
> >>>>> +
> >>>>> + for (int i = 0; i < memsize; i += pagesize)
> >>>>> + memcpy(&mem[i], buf, pagesize);
> >>>>> +
> >>>>> + /* Try and reclaim allocated memory */
> >>>>> + if (cg_write_numeric(cgroup, "memory.reclaim", memsize)) {
> >>>>> + ksft_print_msg("Failed to reclaim all of the requested memory\n");
> >>>>> + goto out;
> >>>>> + }
> >>>>> +
> >>>>> + gettimeofday(&start, NULL);
> >>>>> + /* zswpin */
> >>>>> + for (int i = 0; i < memsize; i += pagesize) {
> >>>>> + if (memcmp(&mem[i], buf, pagesize)) {
> >>>>> + ksft_print_msg("invalid memory\n");
> >>>>> + goto out;
> >>>>> + }
> >>>>> + }
> >>>>> + gettimeofday(&end, NULL);
> >>>>> + printf ("zswapin took %fms to run.\n", (end.tv_sec - start.tv_sec)*1000 + (double)(end.tv_usec - start.tv_usec) / 1000);
> >>>>> + ret = 0;
> >>>>> +out:
> >>>>> + free(mem);
> >>>>> + return ret;
> >>>>> +}
> >>>>> +
> >>>>> +static int test_zswapin_perf(const char *root)
> >>>>> +{
> >>>>> + int ret = KSFT_FAIL;
> >>>>> + char *test_group;
> >>>>> +
> >>>>> + test_group = cg_name(root, "zswapin_perf_test");
> >>>>> + if (!test_group)
> >>>>> + goto out;
> >>>>> + if (cg_create(test_group))
> >>>>> + goto out;
> >>>>> +
> >>>>> + if (cg_run(test_group, zswapin_perf, NULL))
> >>>>> + goto out;
> >>>>> +
> >>>>> + ret = KSFT_PASS;
> >>>>> +out:
> >>>>> + cg_destroy(test_group);
> >>>>> + free(test_group);
> >>>>> + return ret;
> >>>>> +}
> >>>>> +
> >>>>> /*
> >>>>> * When trying to store a memcg page in zswap, if the memcg hits its memory
> >>>>> * limit in zswap, writeback should affect only the zswapped pages of that
> >>>>> @@ -584,6 +654,7 @@ struct zswap_test {
> >>>>> T(test_zswapin),
> >>>>> T(test_zswap_writeback_enabled),
> >>>>> T(test_zswap_writeback_disabled),
> >>>>> + T(test_zswapin_perf),
> >>>>> T(test_no_kmem_bypass),
> >>>>> T(test_no_invasive_cgroup_shrink),
> >>>>> };
> >>>>>
> >>>>> [1] https://lore.kernel.org/all/20241001053222.6944-1-kanchana.p.sridhar@intel.com/
> >>>>> [2] https://lore.kernel.org/all/20240821074541.516249-1-hanchuanhua@oppo.com/
> >>>>> [3] https://lore.kernel.org/all/1505886205-9671-5-git-send-email-minchan@kernel.org/T/#u
> >>>>> [4] https://lwn.net/Articles/955575/
> >>>>>
> >>>>> Usama Arif (4):
> >>>>> mm/zswap: skip swapcache for swapping in zswap pages
> >>>>> mm/zswap: modify zswap_decompress to accept page instead of folio
> >>>>> mm/zswap: add support for large folio zswapin
> >>>>> mm/zswap: count successful large folio zswap loads
> >>>>>
> >>>>> Documentation/admin-guide/mm/transhuge.rst | 3 +
> >>>>> include/linux/huge_mm.h | 1 +
> >>>>> include/linux/zswap.h | 6 ++
> >>>>> mm/huge_memory.c | 3 +
> >>>>> mm/memory.c | 16 +--
> >>>>> mm/page_io.c | 2 +-
> >>>>> mm/zswap.c | 120 ++++++++++++++-------
> >>>>> 7 files changed, 99 insertions(+), 52 deletions(-)
> >>>>>
> >>>>> --
> >>>>> 2.43.5
> >>>>>
> >>>>
> >

Thanks
Barry