[Numpy-discussion] caching large allocations on gnu/linux
faltet at gmail.com
Mon Mar 13 14:54:20 EDT 2017
2017-03-13 18:11 GMT+01:00 Julian Taylor <jtaylor.debian at googlemail.com>:
> On 13.03.2017 16:21, Anne Archibald wrote:
> > On Mon, Mar 13, 2017 at 12:21 PM Julian Taylor
> > <jtaylor.debian at googlemail.com <mailto:jtaylor.debian at googlemail.com>>
> > wrote:
> > Should it be agreed that caching is worthwhile I would propose a very
> > simple implementation. We only really need to cache a small handful
> > array data pointers for the fast allocate deallocate cycle that
> > in common numpy usage.
> > For example a small list of maybe 4 pointers storing the 4 largest
> > recent deallocations. New allocations just pick the first memory
> > of sufficient size.
> > The cache would only be active on systems that support MADV_FREE
> > is linux 4.5 and probably BSD too).
> > So what do you think of this idea?
> > This is an interesting thought, and potentially a nontrivial speedup
> > with zero user effort. But coming up with an appropriate caching policy
> > is going to be tricky. The thing is, for each array, numpy grabs a block
> > "the right size", and that size can easily vary by orders of magnitude,
> > even within the temporaries of a single expression as a result of
> > broadcasting. So simply giving each new array the smallest cached block
> > that will fit could easily result in small arrays in giant allocated
> > blocks, wasting non-reclaimable memory. So really you want to recycle
> > blocks of the same size, or nearly, which argues for a fairly large
> > cache, with smart indexing of some kind.
> The nice thing about MADV_FREE is that we don't need any clever cache.
> The same process that marked the pages free can reclaim them in another
> allocation, at least that is what my testing indicates it allows.
> So a small allocation getting a huge memory block does not waste memory
> as the top unused part will get reclaimed when needed, either by numpy
> itself doing another allocation or a different program on the system.
Well, what you say makes a lot of sense to me, so if you have tested that
then I'd say that this is worth a PR and see how it works on different
> An issue that does arise though is that this memory is not available for
> the page cache used for caching on disk data. A too large cache might
> then be detrimental for IO heavy workloads that rely on the page cache.
Yeah. Also, memory mapped arrays use the page cache intensively, so we
should test this use case and see how the caching affects memory map
> So we might want to cap it to some max size, provide an explicit on/off
> switch and/or have numpy IO functions clear the cache.
allowing the disabling
this feature would be desirable. That would provide an easy path for
testing how it affects performance. Would that be feasible?
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