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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Oct 18, 2016 at 04:20:07PM +0200, Michal Hocko wrote:
> 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.

Well, I don't see a reasonable way to autotune it. We can just let
arch-specific code to redefine it, but the argument below still stands.

> > 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?

There's no fadvise for this at the moment. We can use madvise, except that
the patch makes it lower priority than the limit :P. I'll fix that.

But in general, it would require change to the program which is not always
desirable or even possible.

-- 
 Kirill A. Shutemov

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