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    SubjectRe: [Lse-tech] Re: 10.31 second kernel compile writes:
    > On Sat, Mar 16, 2002 at 01:27:52PM -0700, Richard Gooch wrote:
    > > writes:
    > > > On Sat, Mar 16, 2002 at 09:05:04PM +0100, Andi Kleen wrote:
    > > > > That will hopefully change eventually because 2M pages are a bit help for
    > > > > a lot of applications that are limited by TLB thrashing, but needs some
    > > > > thinking on how to avoid the fragmentation trap (e.g. I'm considering
    > > > > to add a highmem zone again just for that and use rmap with targetted
    > > > > physical freeing to allocating them)
    > > >
    > > > To me, once you have a G of memory, wasting a few meg on unused
    > > > process memory seems no big deal.
    > >
    > > I'm not happy to throw away 2 MiB per process. My workstation has 1
    > > GiB of RAM, and 65 processes (and that's fairly low compared to your
    > > average desktop these days, because I just use olwm and don't have a
    > > fancy desktop or lots of windows). You want me to throw over 1/8th of
    > > my RAM away?!?
    > Why not? If you just ran vim on console you'd be more productive and
    > not need all those worthless processes.

    Yeah, right.

    > At 4KB/page and 8bytes/pte a
    > 1G process will need at least 2MB of pte alone ! Add in the 4 layers,
    > the software VM struct, ...

    This isn't a dedicated bigass-image display box. It's a workstation.
    It's where I read email, hack kernels, write visualisation tools and
    stuff like that.

    And I can afford a few MiB of RAM for PTE's and such for *the one
    process which is mapping my huge data files*! That's effectively a
    small, one-time cost. Every other process doesn't have a significant
    PTE cost.

    I'm not using my kernel as a device driver for an image display
    programme. I'm using it run a box that's generally useful to me.

    > > And in fact, isn't it going to be more than 2 MiB wasted per process?
    > > For each shared object loaded, only partial pages are going to be
    > > used. *My* libc is less than 700 KiB, so I'd be wasting most of a page
    > > to map it in.
    > You're using a politically incorrect libc.

    Yeah :-) Man it feels good.

    > But sure, big pages are not always good.

    Hm. With wide TLB's, what are the benefits to big pages? One
    pathological case that hit me a few years back was a workload which
    bounced around in VM in a pattern that really thrash the cache due to
    aliasing. It wouldn't have been a problem if we had truly fully
    set-associative caches, rather than N-way (where N is 2, 4 or 8
    usually). But big pages won't help that much here (it's just a way of
    reducing TLB thrash, but doesn't help with cache thrashing).


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