linux-kernel - Re: [PATCH] mm: swap: determine swap device by using page nid

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Message-ID: <CAHbLzkoi_a8Tk-bG53cwzcsHhaci30qxi8a4iBXUjVdVc-+kKQ@mail.gmail.com>
Date:   Fri, 29 Apr 2022 12:07:05 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Aaron Lu <aaron.lu@...el.com>
Cc:     "ying.huang@...el.com" <ying.huang@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH] mm: swap: determine swap device by using page nid

On Fri, Apr 29, 2022 at 3:27 AM Aaron Lu <aaron.lu@...el.com> wrote:
>
> On Fri, Apr 22, 2022 at 10:00:59AM -0700, Yang Shi wrote:
> > On Thu, Apr 21, 2022 at 11:24 PM Aaron Lu <aaron.lu@...el.com> wrote:
> > >
> > > On Thu, Apr 21, 2022 at 04:34:09PM +0800, ying.huang@...el.com wrote:
> > > > On Thu, 2022-04-21 at 16:17 +0800, Aaron Lu wrote:
> > > > > On Thu, Apr 21, 2022 at 03:49:21PM +0800, ying.huang@...el.com wrote:
> > >
> > > ... ...
> > >
> > > > > > For swap-in latency, we can use pmbench, which can output latency
> > > > > > information.
> > > > > >
> > > > >
> > > > > OK, I'll give pmbench a run, thanks for the suggestion.
> > > >
> > > > Better to construct a senario with more swapin than swapout.  For
> > > > example, start a memory eater, then kill it later.
> > >
> > > What about vm-scalability/case-swapin?
> > > https://git.kernel.org/pub/scm/linux/kernel/git/wfg/vm-scalability.git/tree/case-swapin
> > >
> > > I think you are pretty familiar with it but still:
> > > 1) it starts $nr_task processes and each mmaps $size/$nr_task area and
> > >    then consumes the memory, after this, it waits for a signal;
> > > 2) start another process to consume $size memory to push the memory in
> > >    step 1) to swap device;
> > > 3) kick processes in step 1) to start accessing their memory, thus
> > >    trigger swapins. The metric of this testcase is the swapin throughput.
> > >
> > > I plan to restrict the cgroup's limit to $size.
> > >
> > > Considering there is only one NVMe drive attached to node 0, I will run
> > > the test as described before:
> > > 1) bind processes to run on node 0, allocate on node 1 to test the
> > >    performance when reclaimer's node id is the same as swap device's.
> > > 2) bind processes to run on node 1, allocate on node 0 to test the
> > >    performance when page's node id is the same as swap device's.
> > >
>
> Thanks to Tim, he has found me a server that has a single Optane disk
> attached to node 0.
>
> Let's use task0_mem0 to denote tasks bound to node 0 and memory bound
> to node 0 through cgroup cpuset. And for the above swapin case:
> when nr_task=1:
> task0_mem0 throughput: [571652, 587158, 594316], avg=584375 -> baseline
> task0_mem1 throughput: [582944, 583752, 589026], avg=585240    +0.15%
> task1_mem0 throughput: [569349, 577459, 581107], avg=575971    -1.4%
> task1_mem1 throughput: [564482, 570664, 571466], avg=568870    -2.6%
>
> task0_mem1 is slightly better than task1_mem0.
>
> For nr_task=8 or nr_task=16, I also gave it a run and the result is
> almost the same for all 4 cases.
>
> > > Ying and Yang,
> > >
> > > Let me know what you think about the case used and the way the test is
> > > conducted.
> >
> > Looks fine to me. To measure the latency, you could also try the below
> > bpftrace script:
> >
>
> Trying to install bpftrace on an old distro(Ubuntu 16.04) is a real
> pain, I gave up... But I managed to get an old bcc installed. Using
> the provided funclatency script to profile 30 seconds swap_readpage(),
> there is no obvious difference from the histrogram.

Thank you so much for the testing.

>
> So for now, from the existing results, it did't show big difference.
> Theoretically, for IO device, when swapping a remote page, using the
> remote swap device that is at the same node as the page can reduce the
> traffic of the interlink and improve performance. I think this is the
> main motivation for this code change?

Yes.

Given the result it seems better to keep the code as-is.

> On swapin time, it's hard to say which node the task will run on anyway
> so it's hard to say where to swap is beneficial.
>