Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn_t
From: John Stoffel
Date: Fri May 08 2015 - 16:42:21 EST
>>>>> "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
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.
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