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SubjectRe: [PATCH -mm 1/2] mm: fix quadratic behaviour in get_unmapped_area_topdown
On Thu, Feb 23, 2012 at 02:56:36PM -0500, Rik van Riel wrote:
> When we look for a VMA smaller than the cached_hole_size, we set the
> starting search address to mm->mmap_base, to try and find our hole.
>
> However, even in the case where we fall through and found nothing at
> the mm->free_area_cache, we still reset the search address to mm->mmap_base.
> This bug results in quadratic behaviour, with observed mmap times of 0.4
> seconds for processes that have very fragmented memory.
>
> If there is no hole small enough for us to fit the VMA, and we have
> no good spot for us right at mm->free_area_cache, we are much better
> off continuing the search down from mm->free_area_cache, instead of
> all the way from the top.

Would it make sense to retain the restart for the case where we _know_
that the remaining address space can not fit the desired area?

/* make sure it can fit in the remaining address space */
if (addr > len) {
vma = find_vma(mm, addr-len);
if (!vma || addr <= vma->vm_start)
/* remember the address as a hint for next time */
return (mm->free_area_cache = addr-len);
} else /* like this */
addr = mm->mmap_base - len;

It would save one pointless find_vma() further down. I don't feel too
strongly about it, though. Either way:

> Signed-off-by: Rik van Riel <riel@redhat.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>


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