Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn_t

[John Stoffel]

>>>>> "Linus" == Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

Linus> On Fri, May 8, 2015 at 7:40 AM, John Stoffel <john@xxxxxxxxxxx> wrote:
>> 
>> Now go and look at your /home or /data/ or /work areas, where the
>> endusers are actually keeping their day to day work.  Photos, mp3,
>> design files, source code, object code littered around, etc.

Linus> However, the big files in that list are almost immaterial from a
Linus> caching standpoint.

Linus> Caching source code is a big deal - just try not doing it and
Linus> you'll figure it out. And the kernel C source files used to
Linus> have a median size around 4k.

Caching any files is a big deal, and if I'm doing batch edits of large
jpegs, won't they get cached as well?   

Linus> The big files in your home directory? Let me make an educated
Linus> guess.  Very few to *none* of them are actually in your page
Linus> cache right now.  And you'd never even care if they ever made
Linus> it into your page cache *at*all*. Much less whether you could
Linus> ever cache them using large pages using some very fancy cache.

Hmm... probably not honestly, since I'm not a home and not using the
system actively right now.  But I can see situations where being able
to mix different page sizes efficiently might be a good thing.  

Linus> There are big files that care about caches, but they tend to be
Linus> binaries, and for other reasons (things like randomization) you
Linus> would never want to use largepages for those anyway.

Or large design files, like my users at $WORK use, which can be 4Gb in
size for a large design, which is ASIC chip layout work.  So I'm a
little bit in the minority there.  

And yes I do have other users will millions of itty bitty files as
well.  

Linus> So from a page cache standpoint, I think the 4kB size still
Linus> matters. A *lot*. largepages are a complete red herring, and
Linus> will continue to be so pretty much forever (anonymous
Linus> largepages perhaps less so).

I think in the future, being able to efficiently mix page sizes will
become useful, if only to lower the memory overhead of keeping track
of large numbers of pages. 

John

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