Messages in this thread | | | Date | Thu, 23 Feb 2012 22:57:23 +0100 | From | Johannes Weiner <> | Subject | Re: [PATCH -mm 1/2] mm: fix quadratic behaviour in get_unmapped_area_topdown |
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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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