From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id B98A0CD1288 for ; Fri, 29 Mar 2024 02:05:47 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 225B86B0089; Thu, 28 Mar 2024 22:05:47 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 1D5B96B008A; Thu, 28 Mar 2024 22:05:47 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 076AA6B008C; Thu, 28 Mar 2024 22:05:47 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id DD6906B0089 for ; Thu, 28 Mar 2024 22:05:46 -0400 (EDT) Received: from smtpin01.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 9E40C40E8B for ; Fri, 29 Mar 2024 02:05:46 +0000 (UTC) X-FDA: 81948435492.01.789C100 Received: from mail-lj1-f172.google.com (mail-lj1-f172.google.com [209.85.208.172]) by imf23.hostedemail.com (Postfix) with ESMTP id D8F87140012 for ; Fri, 29 Mar 2024 02:05:44 +0000 (UTC) Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=google.com header.s=20230601 header.b=tVuoJyt0; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf23.hostedemail.com: domain of yosryahmed@google.com designates 209.85.208.172 as permitted sender) smtp.mailfrom=yosryahmed@google.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1711677945; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=CWzmC2A1K+GOJx5fn1L/D3rGLcK6o/CgnIHLNsZcoy8=; b=jjz+5lZFRo2zyPy3ukOaz/xaGUWiu1zO5nLaf4faSpnHIINJH6Vu41Olv0SaRWmCYrORj/ BYf5Qn75ePXHRpfHcloQYsu6//G3wNmOYNex/pKlsAzKjzO3E4A5qiT9XjUK+oeukNmWLA ucLWQbqoZQBdN+Bvsto8FJtyQYLQ5qo= ARC-Authentication-Results: i=1; imf23.hostedemail.com; dkim=pass header.d=google.com header.s=20230601 header.b=tVuoJyt0; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf23.hostedemail.com: domain of yosryahmed@google.com designates 209.85.208.172 as permitted sender) smtp.mailfrom=yosryahmed@google.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1711677945; a=rsa-sha256; cv=none; b=ruH8ujZ9xbgj885mYbnNJkJC4HgUrSfAGCV2KAzeD1yAA/Khi8Yk9eW2Cx5mHrFONlfPvN yVPEYR0jOv7ksjegvZDZoe73NWuhVu6bOfHbxfqKjbw++FpOxqpnpxZcb7O2UfDNnlgBmZ VLCuw/cSdsVx0XLrh5fxj7cOz23OWvg= Received: by mail-lj1-f172.google.com with SMTP id 38308e7fff4ca-2d718efedb2so592791fa.0 for ; Thu, 28 Mar 2024 19:05:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1711677943; x=1712282743; darn=kvack.org; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=CWzmC2A1K+GOJx5fn1L/D3rGLcK6o/CgnIHLNsZcoy8=; b=tVuoJyt0G4xrL+CaH97GgBcAWlW+CF+m1gmMJ8tnSNiEg3rqzjOqGWRzjyIPmXViCS 0ywVRkjmMTaBH8Kwo/1P+IySeXQ7W93HTLBkx88R/z8IHSojE4/wHjaAEl12KhMet1ef 7w9Md4TRAiZCMIfF/MuzOOKgYF9ueR3vodsi40p0DG+uLzFSp6PbM0OcIuLYNzsDu9bM SW0l9u6i47MxR4hVUci8+b+Ay9N4Wl9G7ZCGonpaclTtsad27CKMda3q2SGUZILoeYWe YEUYSB5kAeEVmHsfMWf0i2VFi7j3ZTbf/e9KqbDG7Dk3MTEqR0hcsI8Toj7Pu26mz1pd 28Mg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1711677943; x=1712282743; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=CWzmC2A1K+GOJx5fn1L/D3rGLcK6o/CgnIHLNsZcoy8=; b=ey1GsR9ik9STClf1Fe9F6ubctHPRIA9msgjKPstW8+c2Xvu75VLr2s14gOJh15OIVU LNOBqBsBQ/if6lg3UsbHMLEqjU5hQ4KJVVfVxdQdtHRT+c7O52Ny6I9+yvcmo7P3QRM9 0NDvrWp7gfBfqHiXjBxhDIpYzqVgt04IJgjqvp/Z5xfYDME5uP/zMb3DzR+KkZ2mkky1 cKojSNM7BXZY8Nxtkt76+C3w/3MJ3vwVEB685lYfnuqeNdf6KdCTWEdyOd0oW6uFIx/C V6UX9DSAsW8OsSNXpuke1DP3iMLBCopaSk2IIQc2KvjjnguTNi9fabRQ9sdk7GsnD7e1 1Cug== X-Forwarded-Encrypted: i=1; AJvYcCURxYf+JW24ReTPkzohJw0jGS8zUGMB99W8SaAJioWlDrjiHBipqvTyk9zYvMtOILds+6C4jpRRSIREiO5HydG1uaA= X-Gm-Message-State: AOJu0Yy+l1HNp5rFT1EKTF3DmbuzRV22jg72UBUltoy23BDqQCxnEpV8 fAbj9WZKRDSpR/SCqSwLrtHqiGEVWlnonUQgNyVQJe2Okb0S04bpchI70QNaVlMQY/e3Y39eAzE ZnhU1PLQgo/HSDD6RxQQDc/4CvYqv6m7DS6Kk X-Google-Smtp-Source: AGHT+IHDTcttH8IbbakiTybsSS1ZVJn3oHaZwKKDQLVTfhzexPbVGNlEI8JbSG2UxzyhRDIqXKr/mAUdBdSj4ROe9rI= X-Received: by 2002:a2e:9659:0:b0:2d6:e148:2463 with SMTP id z25-20020a2e9659000000b002d6e1482463mr586069ljh.24.1711677942540; Thu, 28 Mar 2024 19:05:42 -0700 (PDT) MIME-Version: 1.0 References: <20240325235018.2028408-1-yosryahmed@google.com> <20240325235018.2028408-7-yosryahmed@google.com> <20240328193149.GF7597@cmpxchg.org> <20240328210709.GH7597@cmpxchg.org> In-Reply-To: From: Yosry Ahmed Date: Thu, 28 Mar 2024 19:05:04 -0700 Message-ID: Subject: Re: [RFC PATCH 6/9] mm: zswap: drop support for non-zero same-filled pages handling To: Nhat Pham Cc: Johannes Weiner , Andrew Morton , Chengming Zhou , linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: D8F87140012 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: oawfpikuk31997cdth19jky9h4rcj3pf X-HE-Tag: 1711677944-31612 X-HE-Meta: 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 a5w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Mar 28, 2024 at 4:19=E2=80=AFPM Nhat Pham wrote= : > > On Thu, Mar 28, 2024 at 2:07=E2=80=AFPM Johannes Weiner wrote: > > > > On Thu, Mar 28, 2024 at 01:23:42PM -0700, Yosry Ahmed wrote: > > > On Thu, Mar 28, 2024 at 12:31=E2=80=AFPM Johannes Weiner wrote: > > > > > > > > On Mon, Mar 25, 2024 at 11:50:14PM +0000, Yosry Ahmed wrote: > > > > > The current same-filled pages handling supports pages filled with= any > > > > > repeated word-sized pattern. However, in practice, most of these = should > > > > > be zero pages anyway. Other patterns should be nearly as common. > > > > > > > > > > Drop the support for non-zero same-filled pages, but keep the nam= es of > > > > > knobs exposed to userspace as "same_filled", which isn't entirely > > > > > inaccurate. > > > > > > > > > > This yields some nice code simplification and enables a following= patch > > > > > that eliminates the need to allocate struct zswap_entry for those= pages > > > > > completely. > > > > > > > > > > There is also a very small performance improvement observed over = 50 runs > > > > > of kernel build test (kernbench) comparing the mean build time on= a > > > > > skylake machine when building the kernel in a cgroup v1 container= with a > > > > > 3G limit: > > > > > > > > > > base patched % diff > > > > > real 70.167 69.915 -0.359% > > > > > user 2953.068 2956.147 +0.104% > > > > > sys 2612.811 2594.718 -0.692% > > > > > > > > > > This probably comes from more optimized operations like memchr_in= v() and > > > > > clear_highpage(). Note that the percentage of zero-filled pages d= uring > > > > > this test was only around 1.5% on average, and was not affected b= y this > > > > > patch. Practical workloads could have a larger proportion of such= pages > > > > > (e.g. Johannes observed around 10% [1]), so the performance impro= vement > > > > > should be larger. > > > > > > > > > > [1]https://lore.kernel.org/linux-mm/20240320210716.GH294822@cmpxc= hg.org/ > > > > > > > > > > Signed-off-by: Yosry Ahmed > > > > > > > > This is an interesting direction to pursue, but I actually thinkg i= t > > > > doesn't go far enough. Either way, I think it needs more data. > > > > > > > > 1) How frequent are non-zero-same-filled pages? Difficult to > > > > generalize, but if you could gather some from your fleet, that > > > > would be useful. If you can devise a portable strategy, I'd also= be > > > > more than happy to gather this on ours (although I think you hav= e > > > > more widespread zswap use, whereas we have more disk swap.) > > > > > > I am trying to collect the data, but there are.. hurdles. It would > > > take some time, so I was hoping the data could be collected elsewhere > > > if possible. > > > > > > The idea I had was to hook a BPF program to the entry of > > > zswap_fill_page() and create a histogram of the "value" argument. We > > > would get more coverage by hooking it to the return of > > > zswap_is_page_same_filled() and only updating the histogram if the > > > return value is true, as it includes pages in zswap that haven't been > > > swapped in. > > > > > > However, with zswap_is_page_same_filled() the BPF program will run in > > > all zswap stores, whereas for zswap_fill_page() it will only run when > > > needed. Not sure if this makes a practical difference tbh. > > > > > > > > > > > 2) The fact that we're doing any of this pattern analysis in zswap = at > > > > all strikes me as a bit misguided. Being efficient about repetit= ive > > > > patterns is squarely in the domain of a compression algorithm. D= o > > > > we not trust e.g. zstd to handle this properly? > > > > > > I thought about this briefly, but I didn't follow through. I could tr= y > > > to collect some data by swapping out different patterns and observing > > > how different compression algorithms react. That would be interesting > > > for sure. > > > > > > > > > > > I'm guessing this goes back to inefficient packing from somethin= g > > > > like zbud, which would waste half a page on one repeating byte. > > > > > > > > But zsmalloc can do 32 byte objects. It's also a batching slab > > > > allocator, where storing a series of small, same-sized objects i= s > > > > quite fast. > > > > > > > > Add to that the additional branches, the additional kmap, the ex= tra > > > > scanning of every single page for patterns - all in the fast pat= h > > > > of zswap, when we already know that the vast majority of incomin= g > > > > pages will need to be properly compressed anyway. > > > > > > > > Maybe it's time to get rid of the special handling entirely? > > > > > > We would still be wasting some memory (~96 bytes between zswap_entry > > > and zsmalloc object), and wasting cycling allocating them. This could > > > be made up for by cycles saved by removing the handling. We will be > > > saving some branches for sure. I am not worried about kmap as I think > > > it's a noop in most cases. > > > > Yes, true. > > > > > I am interested to see how much we could save by removing scanning fo= r > > > patterns. We may not save much if we abort after reading a few words > > > in most cases, but I guess we could also be scanning a considerable > > > amount before aborting. On the other hand, we would be reading the > > > page contents into cache anyway for compression, so maybe it doesn't > > > really matter? > > > > > > I will try to collect some data about this. I will start by trying to > > > find out how the compression algorithms handle same-filled pages. If > > > they can compress it efficiently, then I will try to get more data on > > > the tradeoff from removing the handling. > > > > I do wonder if this could be overthinking it, too. > > > > Double checking the numbers on our fleet, a 96 additional bytes for > > each same-filled entry would result in a > > > > 1) p50 waste of 0.008% of total memory, and a > > > > 2) p99 waste of 0.06% of total memory. Right. Assuming the compressors do not surprise us and store same-filled pages in an absurd way, it's not worth it in terms of memory savings. > > > > And this is without us having even thought about trying to make > > zsmalloc more efficient for this particular usecase - which might be > > the better point of attack, if we think it's actually worth it. > > > > So my take is that unless removing it would be outright horrible from > > a %sys POV (which seems pretty unlikely), IMO it would be fine to just > > delete it entirely with a "not worth the maintenance cost" argument. > > > > If you turn the argument around, and somebody would submit the code as > > it is today, with the numbers being what they are above, I'm not sure > > we would even accept it! > > The context guy is here :) > > Not arguing for one way or another, but I did find the original patch > that introduced same filled page handling: > > https://github.com/torvalds/linux/commit/a85f878b443f8d2b91ba76f09da21ac0= af22e07f > > https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d134= 4dde9fce0@epcms5p1/T/#u Thanks for digging this up. I don't know why I didn't start there :) Following in your footsteps, and given that zram has the same feature, I found the patch that added support for non-zero same-filled pages in zram: https://lore.kernel.org/all/1483692145-75357-1-git-send-email-zhouxianrong@= huawei.com/#t Both of them confirm that most same-filled pages are zero pages, but they show a more significant portion of same-filled pages being non-zero (17% to 40%). I suspect this will be less in data centers compared to consumer apps. The zswap patch also reports significant performance improvements from the same-filled handling, but this is with 17-22% same-filled pages. Johannes mentioned around 10% in your data centers, so the performance improvement would be less. In the kernel build tests I ran with only around 1.5% same-filled pages I observed 1.4% improvements just by optimizing them (only zero-filled, skipping allocations). So I think removing the same-filled pages handling completely may be too aggressive, because it doesn't only affect the memory efficiency, but also cycles spent when handling those pages. Just avoiding going through the allocator and compressor has to account for something :) > > The number looks impressive, and there is some detail about the > experiment setup, but I can't seem to find what the allocator + > compressor used. I would assume it was zbud because it was the default then, but as I mentioned zram had similar patches and it uses zsmalloc. I suspect zsmalloc would have changed since then tho, and compressors as well. With zram there, is also the point that metadata is allocated statically AFAICT, so there is very little cost to supporting all same-filled pages if you already support zero-filled pages. > > Which, as Johannes has pointed out, matters a lot. A good compressor > (which should work on arguably the most trivial data pattern there is) > + a backend allocator that is capable of handling small objects well > could make this case really efficient, without resorting to special > handling at the zswap level. All in all, I think there are three aspects here: (1) Cycles saved by avoiding going through the compressor and allocator, and potentially allocating zswap_entry as well the metadata with this series. (2) Memory saved by storing same-filled pages in zswap instead of compressing them. This will ultimately depend on the compressor and allocator as has been established. (3) Memory saved on metadata in zswap by avoiding the zswap_entry allocation. This is probably too small to matter. I think (1) may be the major factor here, and (2) could be a factor if the compressors surprise us (and would always be a factor for zbud and z3fold due to bin packing). Focusing on just (1), supporting zero-filled pages only may be a good tradeoff. We get slightly less complicated and potentially faster handling in zswap without losing a lot. Did I capture the discussion so far correctly?