RE: [PATCH v6 0/3] mm: ZSWAP swap-out of mTHP folios

Sridhar, Kanchana P
2024-09-20T16:53:16Z

> -----Original Message-----
> From: Huang, Ying <ying.huang@intel.com>
> Sent: Friday, September 20, 2024 2:12 AM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> Cc: Nhat Pham <nphamcs@gmail.com>; Yosry Ahmed
> <yosryahmed@google.com>; linux-kernel@vger.kernel.org; linux-
> mm@kvack.org; hannes@cmpxchg.org; chengming.zhou@linux.dev;
> usamaarif642@gmail.com; ryan.roberts@arm.com; 21cnbao@gmail.com;
> akpm@linux-foundation.org; Zou, Nanhai <nanhai.zou@intel.com>; Feghali,
> Wajdi K <wajdi.k.feghali@intel.com>; Gopal, Vinodh
> <vinodh.gopal@intel.com>
> Subject: Re: [PATCH v6 0/3] mm: ZSWAP swap-out of mTHP folios
>
> "Sridhar, Kanchana P" <kanchana.p.sridhar@intel.com> writes:
>
> > Hi Nhat,
> >
> >> -----Original Message-----
> >> From: Nhat Pham <nphamcs@gmail.com>
> >> Sent: Thursday, August 29, 2024 4:46 PM
> >> To: Yosry Ahmed <yosryahmed@google.com>
> >> Cc: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>; linux-
> >> kernel@vger.kernel.org; linux-mm@kvack.org; hannes@cmpxchg.org;
> >> chengming.zhou@linux.dev; usamaarif642@gmail.com;
> >> ryan.roberts@arm.com; Huang, Ying <ying.huang@intel.com>;
> >> 21cnbao@gmail.com; akpm@linux-foundation.org; Zou, Nanhai
> >> <nanhai.zou@intel.com>; Feghali, Wajdi K <wajdi.k.feghali@intel.com>;
> >> Gopal, Vinodh <vinodh.gopal@intel.com>
> >> Subject: Re: [PATCH v6 0/3] mm: ZSWAP swap-out of mTHP folios
> >>
> >> On Thu, Aug 29, 2024 at 3:49 PM Yosry Ahmed
> <yosryahmed@google.com>
> >> wrote:
> >> >
> >> > On Thu, Aug 29, 2024 at 2:27 PM Kanchana P Sridhar
> >> >
> >> > We are basically comparing zram with zswap in this case, and it's not
> >> > fair because, as you mentioned, the zswap compressed data is being
> >> > accounted for while the zram compressed data isn't. I am not really
> >> > sure how valuable these test results are. Even if we remove the cgroup
> >> > accounting from zswap, we won't see an improvement, we should
> expect a
> >> > similar performance to zram.
> >> >
> >> > I think the test results that are really valuable are case 1, where
> >> > zswap users are currently disabling CONFIG_THP_SWAP, and get to
> enable
> >> > it after this series.
> >>
> >> Ah, this is a good point.
> >>
> >> I think the point of comparing mTHP zswap v.s mTHP (SSD)swap is more
> >> of a sanity check. IOW, if mTHP swap outperforms mTHP zswap, then
> >> something is wrong (otherwise why would enable zswap - might as well
> >> just use swap, since SSD swap with mTHP >>> zswap with mTHP >>>
> zswap
> >> without mTHP).
> >>
> >> That said, I don't think this benchmark can show it anyway. The access
> >> pattern here is such that all the allocated memories are really cold,
> >> so swap to disk (or to zram, which does not account memory usage
> >> towards cgroup) is better by definition... And Kanchana does not seem
> >> to have access to setup with larger SSD swapfiles? :)
> >
> > As follow up, I created a swapfile on disk to increase the SSD swap to 179G.
>
> Are you sure you used swapfile instead of a swap partition? From the
> following code in scan_swap_map_slots(),
>
> if (order > 0) {
> /*
> * Should not even be attempting large allocations when huge
> * page swap is disabled. Warn and fail the allocation.
> */
> if (!IS_ENABLED(CONFIG_THP_SWAP) ||
> nr_pages > SWAPFILE_CLUSTER) {
> VM_WARN_ON_ONCE(1);
> return 0;
> }
>
> /*
> * Swapfile is not block device or not using clusters so unable
> * to allocate large entries.
> */
> if (!(si->flags & SWP_BLKDEV) || !si->cluster_info)
> return 0;
> }
>
> large folio will be split for swapfile.

I see. Thanks for this clarification. No, this is a configuration with
175G swapfile on disk + 4G SSD. Large folios being split for swapfile
probably explains the memcg_swap_fail counts in this case.

Thanks,
Kanchana

>
> --
> Best Regards,
> Huang, Ying
>
> > 64KB mTHP (cgroup memory.high set to 40G, no swap limit):
> > =========================================================
> > CONFIG_THP_SWAP=Y
> > Sapphire Rapids server with 503 GiB RAM and 179G SSD swap backing
> device
> > for zswap.
> >
> > usemem --init-time -w -O --sleep 0 -n 70 1g:
> >
> > -------------------------------------------------------------------------------
> > mm-unstable 9-17-2024 zswap-mTHP v6 Change wrt
> > Baseline Baseline
> > "before" "after" (sleep 0)
> > -------------------------------------------------------------------------------
> > ZSWAP compressor zstd deflate- zstd deflate- zstd deflate-
> > iaa iaa iaa
> > -------------------------------------------------------------------------------
> > Throughput (KB/s) 93,273 88,496 143,117 134,131 53% 52%
> > sys time (sec) 316.68 349.00 917.88 877.74 -190% -152%
> > memcg_high 73,836 83,522 126,120 133,013
> > memcg_swap_fail 261,136 324,533 494,191 578,824
> > pswpin 16 11 0 0
> > pswpout 1,242,187 1,263,493 0 0
> > zswpin 694 668 712 702
> > zswpout 3,991,403 4,933,901 9,289,092 10,461,948
> > thp_swpout 0 0 0 0
> > thp_swpout_ 0 0 0 0
> > fallback
> > pgmajfault 3,488 3,353 3,377 3,499
> > ZSWPOUT-64kB n/a n/a 110,067 103,957
> > SWPOUT-64kB 77,637 78,968 0 0
> > -------------------------------------------------------------------------------
> >
> > We do see 50% throughput improvement with mTHP-zswap wrt mTHP-SSD.
> > The sys time increase can be attributed to higher swapout activity
> > occurring with zswap-mTHP.
> >
> > I hope this quantifies the benefit of mTHP-zswap wrt mTHP-SSD in a
> > non-swap-constrained setup. The 4G SSD swap setup data I shared
> > in my response to Yosry also indicates better throughput with mTHP-zswap
> > as compared to mTHP-SSD.
> >
> > Please do let me know if you have any other questions/suggestions.
> >
> > Thanks,
> > Kanchana
> >
> >>
> >> >
> >> > If we really want to compare CONFIG_THP_SWAP on before and after, it
> >> > should be with SSD because that's a more conventional setup. In this
> >> > case the users that have CONFIG_THP_SWAP=y only experience the
> >> > benefits of zswap with this series. You mentioned experimenting with
> >> > usemem to keep the memory allocated longer so that you're able to have
> >> > a fair test with the small SSD swap setup. Did that work?
> >> >
> >> > I am hoping Nhat or Johannes would shed some light on whether they
> >> > usually have CONFIG_THP_SWAP enabled or not with zswap. I am trying
> to
> >> > figure out if any reasonable setups enable CONFIG_THP_SWAP with
> zswap.
> >> > Otherwise the testing results from case 1 should be sufficient.
> >> >
> >> > >
> >> > > In my opinion, even though the test set up does not provide an
> accurate
> >> > > way for a direct before/after comparison (because of zswap usage
> being
> >> > > counted in cgroup, hence towards the memory.high), it still seems
> >> > > reasonable for zswap_store to support (m)THP, so that further
> >> performance
> >> > > improvements can be implemented.
> >> >
> >> > This is only referring to the results of case 2, right?
> >> >
> >> > Honestly, I wouldn't want to merge mTHP swapout support on its own
> >> > just because it enables further performance improvements without
> >> > having actual patches for them. But I don't think this captures the
> >> > results accurately as it dismisses case 1 results (which I think are
> >> > more reasonable).
> >> >
> >> > Thnaks