Messages in this thread | | | Date | Mon, 09 Mar 2009 21:45:11 +0200 | From | Avi Kivity <> | Subject | Re: [patch] aio: remove aio-max-nr and instead use the memlock rlimit to limit the number of pages pinned for the aio completion ring |
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Jeff Moyer wrote: >> Is it not possible to get rid of the pinning entirely? Pinning >> interferes with page migration which is important for NUMA, among >> other issues. >> > > aio_complete is called from interrupt handlers, so can't block faulting > in a page. Zach mentions there is a possibility of handing completions > off to a kernel thread, with all of the performance worries and extra > bookkeeping that go along with such a scheme (to help frame my concerns, > I often get lambasted over .5% performance regressions). >
Or you could queue the completions somewhere, and only copy them to user memory when io_getevents() is called. I think the plan was once to allow events to be consumed opportunistically even without io_getevents(), though.
> I'm happy to look into such a scheme, should anyone show me data that > points to this NUMA issue as an actual performance problem today. In > the absence of such data, I simply can't justify the work at the moment. >
Right now page migration is a dead duck. Outside HPC, there is now support for triggering it or for getting the scheduler to prefer a process's memory node. Only a minority of hosts are NUMA.
I think that will/should change in the near future. Nehalem-based servers mean that NUMA will be commonplace. The larger core counts will mean that hosts will run several unrelated applications (often through virtualization); such partitioning can easily benefit from page migration.
> Thanks for taking a look! >
Sorry, I didn't actually take a look at the patches. I only reacted to the description - I am allergic to pinned memory.
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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