linux-kernel - Re: Re: [PATCH V2] kmalloc_index optimization(code size & runtime stable)

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Date:   Tue, 21 Apr 2020 07:36:09 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     赵军奎 <bernard@...o.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: Re: [PATCH V2] kmalloc_index optimization(code size & runtime
 stable)

On Tue, Apr 21, 2020 at 07:55:03PM +0800, 赵军奎 wrote:
> Sure, i just received some kbuild compiler error mails and prompt me to do something? 
> I don`t know why this happened, so i update the patch again.

Don't.  The patch has been NACKed, so there's no need to post a v2.

If you want to do something useful, how about looking at the effect
of adding different slab sizes?  There's a fairly common pattern of
allocating things which are a power of two + a header.  So it may make
sense to have kmalloc caches of 320 (256 + 64), 576 (512 + 64) and 1088
(1024 + 64).  I use 64 here as that's the size of a cacheline, so we
won't get false sharing between users.

This could save a fair quantity of memory; today if you allocate 512 +
8 bytes, it will round up to 1024.  So we'll get 4 allocations per 4kB
page, but with a 576-byte slab, we'd get 7 allocations per 4kB page.
Of course, if there aren't a lot of users which allocate memory in this
range, then it'll be a waste of memory.  On my laptop, it seems like
there might be a decent amount of allocations in the right range:

kmalloc-2k          3881   4384   2048   16    8 : tunables    0    0    0 : sla
bdata    274    274      0
kmalloc-1k          6488   7056   1024   16    4 : tunables    0    0    0 : slabdata    441    441      0
kmalloc-512         7700   8256    512   16    2 : tunables    0    0    0 : slabdata    516    516      0

Now, maybe 576 isn't quite the right size.  Need to try it on a variety
of configurations and find out.  Want to investigate this?