linux-kernel - Re: [PATCH 4/4] workqueue: slash half memory usage in 32bit system

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Message-ID: <CAJhGHyCn4R-0rv2bDBWuYs1SE0PyJe8GoMEhHo4xwtUy23YpWg@mail.gmail.com>
Date:   Tue, 2 Jun 2020 08:08:10 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] workqueue: slash half memory usage in 32bit system

On Mon, Jun 1, 2020 at 11:07 PM Tejun Heo <tj@...nel.org> wrote:
>
> On Mon, Jun 01, 2020 at 08:44:42AM +0000, Lai Jiangshan wrote:
> > The major memory ussage in workqueue is on the pool_workqueue.
> > The pool_workqueue has alignment requirement which often leads
> > to padding.
> >
> > Reducing the memory usage for the pool_workqueue is valuable.
> >
> > And 32bit system often has less memory, less workqueues,
> > less works, less concurrent flush_workqueue()s, so we can
> > slash the flush color on 32bit system to reduce memory usage
> >
> > Before patch:
> > The sizeof the struct pool_workqueue is 256 bytes,
> > only 136 bytes is in use in 32bit system
> >
> > After patch:
> > The sizeof the struct pool_workqueue is 128 bytes,
> > only 104 bytes is in use in 32bit system, there is still
> > room for future usage.
> >
> > Setting WORK_STRUCT_COLOR_BITS to 3 can't reduce the sizeof
> > the struct pool_workqueue in 64bit system, unless combined
> > with big refactor for unbound pwq.
>
> Have you calculated how much memory is actually saved this way on a typical
> system?

It is not noticable from the "free" command.
By counting the number of allocated pwq (mainly percpu pwq),
it saves 20k in my simple kvm guest (4cpu).
I guess it highly various in different boxes with various
kernel modules loaded.

>
> Thanks.
>
> --
> tejun