linux-kernel - Re: [PATCH] swap: Add percpu cluster_next to reduce lock contention on swap cache

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Date:   Mon, 18 May 2020 21:39:30 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
        Minchan Kim <minchan@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] swap: Add percpu cluster_next to reduce lock contention
 on swap cache

On Mon, May 18, 2020 at 02:37:15PM +0800, Huang, Ying wrote:
> Daniel Jordan <daniel.m.jordan@...cle.com> writes:
> > On Thu, May 14, 2020 at 03:04:24PM +0800, Huang Ying wrote:
> >> And the pmbench score increases 15.9%.
> >
> > What metric is that, and how long did you run the benchmark for?
> 
> I run the benchmark for 1800s.  The metric comes from the following
> output of the pmbench,
> 
> [1] Benchmark done - took 1800.088 sec for 122910000 page access
> 
> That is, the throughput is 122910000 / 1800.088 = 68280.0 (accesses/s).
> Then we sum the values from the different processes.

Ok.

> > It's just a nit but SWP_SOLIDSTATE and 'if (si->cluster_info)' are two ways to
> > check the same thing and I'd stick with the one that's already there.
> 
> Yes.  In effect, (si->flags & SWP_SOLIDSTATE) and (si->cluster_info)
> always has same value at least for now.  But I don't think they are
> exactly same in semantics.  So I would rather to use their exact
> semantics.

Oh, but I thought the swap clusters were for scaling the locking for fast
devices, so that both checks have the same semantics now, and presumably would
in the future.

It's a minor point, I'm fine either way.

> The first swap slot is the swap partition header, you cand find the
> corresponding code in syscall swapon function, below comments "Read the

Aha, thanks.