linux-kernel - Re: [PATCH] shmem: avoid huge pages for small files

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Date:   Tue, 18 Oct 2016 16:20:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] shmem: avoid huge pages for small files

On Mon 17-10-16 17:55:40, Kirill A. Shutemov wrote:
> On Mon, Oct 17, 2016 at 04:12:46PM +0200, Michal Hocko wrote:
> > On Mon 17-10-16 15:30:21, Kirill A. Shutemov wrote:
[...]
> > > We add two handle to specify minimal file size for huge pages:
> > > 
> > >   - mount option 'huge_min_size';
> > > 
> > >   - sysfs file /sys/kernel/mm/transparent_hugepage/shmem_min_size for
> > >     in-kernel tmpfs mountpoint;
> > 
> > Could you explain who might like to change the minimum value (other than
> > disable the feautre for the mount point) and for what reason?
> 
> Depending on how well CPU microarchitecture deals with huge pages, you
> might need to set it higher in order to balance out overhead with benefit
> of huge pages.

I am not sure this is a good argument. How do a user know and what will
help to make that decision? Why we cannot autotune that? In other words,
adding new knobs just in case turned out to be a bad idea in the past.

> In other case, if it's known in advance that specific mount would be
> populated with large files, you might want to set it to zero to get huge
> pages allocated from the beginning.

Cannot we use [mf]advise for that purpose?
-- 
Michal Hocko
SUSE Labs