Re: missing madvise functionality

Nick Piggin
2007-04-06T02:59:42Z

Ulrich Drepper wrote:
> Nick Piggin wrote:
>
>>Cool. According to my thinking, madvise(MADV_DONTNEED) even in today's
>>kernels using down_write(mmap_sem) for MADV_DONTNEED is better than
>>mmap/mprotect, which have more fundamental locking requirements, more
>>overhead and no benefits (except debugging, I suppose).
>
>
> It's a tiny bit faster, see
>
> http://people.redhat.com/drepper/dontneed.png
>
> I just ran it once so the graph is not smooth. This is on a UP dual
> core machine. Maybe tomorrow I'll turn on the big 4p machine.

Hmm, I saw an improvement, but that was just on a raw syscall test
with a single page chunk. Real-world use I guess will get progressively
less dramatic as other overheads start being introduced.

Multi-thread performance probably won't get a whole lot better (it does
eliminate 1 down_write(mmap_sem), but one remains) until you use my
madvise patch.

> I would have to see dramatically different results on the big machine to
> make me change the libc code. The reason is that there is a big drawback.
>
> So far, when we allocate a new arena, we allocate address space with
> PROT_NONE and only when we need memory the protection is changed to
> PROT_READ|PROT_WRITE. This is the advantage of catching wild pointer
> accesses.

Sure, yes. And I guess you'd always want to keep that options around as
a debugging aid.

--
SUSE Labs, Novell Inc.