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

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

> -----Original Message-----
> From: Yosry Ahmed <yosryahmed@google.com>
> Sent: Thursday, August 29, 2024 5:14 PM
> To: Nhat Pham <nphamcs@gmail.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 5:06 PM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > On Thu, Aug 29, 2024 at 4:55 PM Yosry Ahmed <yosryahmed@google.com>
> wrote:
> > >
> > > On Thu, Aug 29, 2024 at 4:45 PM Nhat Pham <nphamcs@gmail.com>
> wrote:
> > > I think it's also the fact that the processes exit right after they
> > > are done allocating the memory. So I think in the case of SSD, when we
> > > stall waiting for IO some processes get to exit and free up memory, so
> > > we need to do less swapping out in general because the processes are
> > > more serialized. With zswap, all processes try to access memory at the
> > > same time so the required amount of memory at any given point is
> > > higher, leading to more thrashing.
> > >
> > > I suggested keeping the memory allocated for a long time to even the
> > > playing field, or we can make the processes keep looping and accessing
> > > the memory (or part of it) for a while.
> > >
> > > That being said, I think this may be a signal that the memory.high
> > > throttling is not performing as expected in the zswap case. Not sure
> > > tbh, but I don't think SSD swap should perform better than zswap in
> > > that case.
> >
> > Yeah something is fishy there. That said, the benchmarking in v4 is wack:
> >
> > 1. We use lz4, which has a really poor compression factor.
> >
> > 2. The swapfile is really small, so we occasionally see problems with
> > swap allocation failure.
> >
> > Both of these factors affect benchmarking validity and stability a
> > lot. I think in this version's benchmarks, with zstd as the software
> > compressor + a much larger swapfile (albeit on top of a ZRAM block
> > device), we no longer see memory.high violation, even at a lower
> > memory.high value...? The performance number is wack indeed - not a
> > lot of values in the case 2 section.
> 
> But when we use zram we are essentially comparing two swap mechanisms
> compressing mTHPs page by page, with the only difference being that
> zram does not account the memory. For this to have any value imo it
> should be on an SSD to at least provide the value of being a practical
> sanity check as you mentioned earlier. In its current form I don't
> think it's providing any value.

Just posted data today with SSD and longer running usemem processes,
that should hopefully better quantify the benefit of zswap-mTHP.

Thanks,
Kanchana