Messages in this thread | | | Date | Fri, 27 Apr 2007 21:05:17 +1000 | From | Paul Mackerras <> | Subject | Re: [00/17] Large Blocksize Support V3 |
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Andrew Morton writes:
> If x86 had larger pagesize we wouldn't be seeing any of this. It is a workaround > for present-generation hardware.
Unfortunately, it's not really practical to increase the page size very much on most systems, because you end up wasting a lot of space in the page cache. So there is a tension between wanting a small page size so your page cache uses memory efficiently, and wanting a large page size so the TLB covers more address space and your programs run faster (not to mention other benefits such as the kernel having to manage fewer pages, and I/O being done in bigger chunks).
Thus there is not really any single page size that suits all workloads and machines. With distros wanting to just have a single kernel per architecture, and the fact that the page size is a compile-time constant, we currently end up having to pick one size and just put up with the fact that it will suck for some users. We currently have this situation on ppc64 now that POWER5+ and POWER6 machines have hardware support for 64k pages as well as 4k pages.
So I can see a few different options:
(a) Keep things more or less as they are now and just wear the fact that we will continue to show lower performance than certain proprietary OSes, or
(b) Somehow manage to make the page size a variable rather than a compile-time constant, and pick a suitable page size at boot time based on how much memory the machine has, or something. I looked at implementing this at one point and recoiled in horror. :)
(c) Make the page cache able to use small pages for small files and large pages for large files. AIUI this is basically what Christoph is proposing.
Option (a) isn't very palatable to me (nor I expect, Christoph :) since it basically says that Linux is very much focussed on the embedded and desktop end of things and isn't really suitable as a high-performance OS for large SMP systems. I don't want to believe that. ;)
Option (b) would be a bit of an ugly hack.
Which leaves option (c) - unless you have a further option. So I have to say I support Christoph on this, at least as far as the general principle is concerned.
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