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SubjectRe: [PATCH -next 0/2] fs/epoll: loosen irq safety when possible
On Fri, 20 Jul 2018, Andrew Morton wrote:

>Did you try measuring it on bare hardware?

I did and wasn't expecting much difference.

For a 2-socket 40-core (ht) IvyBridge on a few workloads, unfortunately
I don't have a xen environment and the results for Xen I do have (which numbers
are in patch 1) I don't have the actual workload, so cannot compare them directly.

1) Different configurations were used for a epoll_wait (pipes io) microbench
(http://linux-scalability.org/epoll/epoll-test.c) and shows around a 7-10%
improvement in overall total number of times the epoll_wait() loops when using
both regular and nested epolls, so very raw numbers, but measurable nonetheless.

# threads vanilla dirty
1 1677717 1805587
2 1660510 1854064
4 1610184 1805484
8 1577696 1751222
16 1568837 1725299
32 1291532 1378463
64 752584 787368

Note that stddev is pretty small.

2) Another pipe test, which shows no real measurable improvement.
(http://www.xmailserver.org/linux-patches/pipetest.c)

>> >
>> >I'd have more confidence if we had some warning mechanism if we run
>> >spin_lock_irq() when IRQs are disabled, which is probably-a-bug. But
>> >afaict we don't have that. Probably for good reasons - I wonder what
>> >they are?
>
>Well ignored ;)
>
>We could open-code it locally. Add a couple of
>WARN_ON_ONCE(irqs_disabled())? That might need re-benchmarking with
>Xen but surely just reading the thing isn't too expensive?

I agree, I'll see what I can come up with and also ask the customer to test
in his setup. Bare metal would also need some new numbers I guess.

Thanks,
Davidlohr

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