Re: [BProc] vmadump performance

[Erik A. H. <er...@he...>]

On Thu, Jun 06, 2002 at 04:44:47PM -0400, Grant Taylor wrote:
> >>>>> Erik Arjan Hendriks <er...@he...> writes:
> >>  - Removing seemingly uneeded icache flushes for non-executable pages
> >>    in load_map makes no difference, time-wise, on my platform.  This
> 
> > That's good.  I don't find it surprising though since most of the
> > addresses involved in the flush aren't going to be in the icache
> 
> OK, I lied.  Evidently that experiment was done with the wrong kernel
> or something.  In fact it's like 5X faster, as cache fluses are fairly
> expensive on my (SMP) platform.  Profile below.
> 
> So you definitely want to stick an if around the flush_icache_range
> call for the benefit of those of us with expensive ipi cache
> nastiness:
> 
>       if (mmap_prot & PROT_EXEC) {
>         flush_icache_range(page.start, page.start + PAGE_SIZE);
>       }

Cool.  Patch added.

> I also observe in my profile that the read's memcpy ("both_aligned" in
> my profile) and the fault's memzero ("sb1_clear_page") each take
> nearly half the time.  I'm going to bodge together some way to skip
> the page clear for this case; we already know we're overwriting the
> whole page, so this should be OK as long as it gets carefully zeroed
> on error.  It'll be pretty ugly, though, so I suspect you won't
> actually want it.

[snip]

I find it surprising that the cache stuff is so expensive.  Is the
hardware somewhat lacking in maintaining coherency between CPUs?

- Erik