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SubjectRe: [patch] aio: remove aio-max-nr and instead use the memlock rlimit to limit the number of pages pinned for the aio completion ring
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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