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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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