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

["Sridhar, Kanchana P" <kanchana.p.sridhar@intel.com>]

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.

 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