Messages in this thread | | | From | Al Boldi <> | Subject | Re: How can we make page replacement smarter | Date | Sun, 29 Jul 2007 17:55:24 +0300 |
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Rik van Riel wrote: > Al Boldi wrote: > > Good idea, but unless we understand the problems involved, we are bound > > to repeat it. So my first question would be: Why is swap-in so slow? > > > > As I have posted in other threads, swap-in of consecutive pages suffers > > a 2x slowdown wrt swap-out, whereas swap-in of random pages suffers over > > 6x slowdown. > > > > Because it is hard to quantify the expected swap-in speed for random > > pages, let's first tackle the swap-in of consecutive pages, which should > > be at least as fast as swap-out. So again, why is swap-in so slow? > > I suspect that this is a locality of reference issue. > > Anonymous memory can get jumbled up by repeated free and > malloc cycles of many smaller objects. The amount of > anonymous memory is often smaller than or roughly the same > size as system memory.
Sounds exactly like the tmpfs problem.
> Locality of refenence to anonymous memory tends to be > temporal in nature, with the same sets of pages being > accessed over and over again. > > Files are different. File content tends to be grouped > in large related chunks, both logically in the file and > on disk. Generally there is a lot more file data on a > system than what fits in memory. > > Locality of reference to file data tends to be spatial > in nature, with one file access leading up to the system > accessing "nearby" data. The data is not necessarily > touched again any time soon. > > > Once we understand this problem, we may be able to suggest a smart > > improvement. > > Like the one on http://linux-mm.org/PageoutFailureModes ?
Interesting to see that there are known problems, but it doesn't seem to list the resume-from-disk swap-in slowdown.
> I have the LRU lists split and am working on getting SEQ > replacement implemented for the anonymous pages. > > The most recent (untested) patches are attached.
Applied against 2.6.22; the kernel crashes out on boot.
Thanks!
-- Al
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