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Tue, 17 Feb 2026 15:36:34 -0800 (PST) MIME-Version: 1.0 References: <20260208215839.87595-2-nphamcs@gmail.com> <20260208222652.328284-1-nphamcs@gmail.com> In-Reply-To: From: Nhat Pham Date: Tue, 17 Feb 2026 15:36:23 -0800 X-Gm-Features: AaiRm50S_8HbiTq-rHvY-Vu5YZbVnz6yG-KBkKAQGni1VF15QKHPJ6JJsJaTSsw Message-ID: Subject: Re: [PATCH v3 00/20] Virtual Swap Space To: Kairui Song Cc: linux-mm@kvack.org, akpm@linux-foundation.org, hannes@cmpxchg.org, hughd@google.com, yosry.ahmed@linux.dev, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, len.brown@intel.com, chengming.zhou@linux.dev, chrisl@kernel.org, huang.ying.caritas@gmail.com, ryan.roberts@arm.com, shikemeng@huaweicloud.com, viro@zeniv.linux.org.uk, baohua@kernel.org, bhe@redhat.com, osalvador@suse.de, christophe.leroy@csgroup.eu, pavel@kernel.org, kernel-team@meta.com, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-pm@vger.kernel.org, peterx@redhat.com, riel@surriel.com, joshua.hahnjy@gmail.com, npache@redhat.com, gourry@gourry.net, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, rafael@kernel.org, jannh@google.com, pfalcato@suse.de, zhengqi.arch@bytedance.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: rx7zqrkkohg913qkd6orsywwuuhpfkk7 X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: C9BE710000A X-HE-Tag: 1771371396-420563 X-HE-Meta: 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 4+iLerBs AAI4VOf2qUYsVaXOQ0x1Gp2HkO29Fv8VBsIkEMGH3FcW/HLwk04blzSvgqxiZqxFPf72zvi684agGPDh5RZNTna4DNgg3q667TPideUGJs2mgoYcIKbUDsJfhlaLAX3TB+W6dRnwS56RObAtDIyGkSfn0UviYui8gQM4cxVCNxTYue71bqB+8Qmr2dOc77HP9sIf/8X3q16JEJbckx5BB3EatyYlh3fZDS9XTv25Wd0kEqGRgi9n2oJ0SnDm05MHEKiATxPbm9p5nFI5tvwpd46t2ztUrXv21Ur7uYn5Ihci6nRzyVY9cEWg4l/2Hvjhfs7m/WM0dAhlONfo8d39fMzQshRnyv0KL6g/FYEeUIKXOLL6a2lw27kcYCanLJw+0WJhrpIh7G8rIq0+YXdQS+/9+y6AXaW1L6ETWdrocRAmCFCHacCCmRb+Ede7SH2ips6ySspGs4hmEWGnI/dSvLIfNMUZyS1XbXArotVFlv0U+zXlux7ycz+ZnwQtTkKI37mVyTnVk5K/NDOxT1Zmya51wakIBbIpxjN2T 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 Tue, Feb 10, 2026 at 11:11=E2=80=AFAM Nhat Pham wrot= e: > > On Tue, Feb 10, 2026 at 10:00=E2=80=AFAM Kairui Song w= rote: > > # free -m > > total used free shared buff/cache a= vailable > > Mem: 31582 909 26388 8 4284 = 29989 > > Swap: 40959 41 40918 > > > > The swap setup follows the recommendation from Huang > > (https://lore.kernel.org/linux-mm/87ed474kvx.fsf@yhuang6-desk2.ccr.corp= .intel.com/). > > > > Test (average of 18 test run): > > vm-scalability/usemem --init-time -O -y -x -n 1 56G > > > > 6.19: > > Throughput: 618.49 MB/s (stdev 31.3) > > Free latency: 5754780.50us (stdev 69542.7) > > > > swap-table-p3 (3.8%, 0.5% better): > > Throughput: 642.02 MB/s (stdev 25.1) > > Free latency: 5728544.16us (stdev 48592.51) > > > > vswap (3.2%, 244% worse): > > Throughput: 598.67 MB/s (stdev 25.1) > > Free latency: 13987175.66us (stdev 125148.57) > > > > That's a huge regression with freeing. I have a vm-scatiliby test > > matrix, not every setup has such significant >200% regression, but on > > average the freeing time is about at least 15 - 50% slower (for > > example /data/vm-scalability/usemem --init-time -O -y -x -n 32 1536M > > the regression is about 2583221.62us vs 2153735.59us). Throughput is > > all lower too. Hi Kairui - a quick update. Took me awhile to get a host that matches your memory spec: free -m total used free shared buff/cache avai= lable Mem: 31609 5778 7634 20 18664 = 25831 Swap: 65535 1 65534 I think I managed to reproduce your observations (average over 5 runs): Baseline (6.19) real: mean: 191.19s, stdev: 4.53s user: mean: 46.98s, stdev: 0.15s sys: mean: 127.97s, stdev: 3.95s average throughput: 382057 KB/s average free time: 8179978 usecs Vswap: real: mean: 199.85s, stdev: 6.09s user: mean: 46.51s, stdev: 0.25s sys: mean: 137.24s, stdev: 6.46s average throughput: 367437 KB/s average free time: 9887107.6 usecs (command is time ./usemem --init-time -w -O -s 10 -n 1 56g) I think I figured out where the bulk of the regression lay - it's in the PTE zapping path. In a nutshell, we're not batching in the case where these PTEs are backed by virtual swap entries with zswap backends (even though there is not a good reason not to batch), and unnecessarily performing unnecesary xarray lookups to resolve the backend for some superfluous checks (2 xarray lookups for every PTE, which is wasted work because as noted earlier, we ended up not batching anyway). Just by simply fixing this issue, the gap is much closer real: mean: 192.24s, stdev: 4.82s user: mean: 46.42s, stdev: 0.27s sys: mean: 129.84s, stdev: 4.59s average throughput: 380670 KB/s average free time: 8583381.4 usecs I also discovered a couple more inefficiencies in vswap free path. Hopefully once we fix those, the gap will be non-existent.