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    SubjectRe: [00/17] Large Blocksize Support V3
    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.

    Regards,
    Paul.
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