Re: missing madvise functionality

[Ulrich Drepper]

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.

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.

-- 
â Ulrich Drepper â Red Hat, Inc. â 444 Castro St â Mountain View, CA â

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