Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table

[Avi Kivity <avi@cloudius-systems.com>]

On 11/28/2013 12:40 PM, Paolo Bonzini wrote:
> Il 28/11/2013 11:16, Avi Kivity ha scritto:
>>> The QRCU I linked would work great latency-wise (it has roughly the same
>>> latency of an rwsem but readers are lock-free).  However, the locked
>>> operations in the read path would hurt because of cache misses, so it's
>>> not good either.
>> I guess srcu would work.  Do you know what's the typical write-side
>> latency there? (in terms of what it waits for, not nanoseconds).
> If there's no concurrent reader, it's zero if I read the code right.
> Otherwise it depends:
>
> - if there are many callbacks, only 10 of them are processed per
> millisecond.  But unless there are concurrent synchronize_srcu calls
> there should not be any callback at all.  If all VCPUs were to furiously
> change the MSIs, the latency could go up to #vcpu/10 milliseconds.
>
> - if there are no callbacks, but there are readers, synchronize_srcu
> busy-loops for some time checking if the readers complete.  After a
> while (20 us for synchronize_srcu, 120 us for
> synchronize_srcu_expedited) it gives up and starts using a workqueue to
> poll every millisecond.  This should never happen unless
>
> So, given the very restricted usage this SRCU would have, we probably
> can expect synchronize_srcu_expedited to finish its job in the
> busy-looping phase, and 120 us should be the expected maximum
> latency---more likely to be an order of magnitude smaller, and in very
> rare cases higher.
>

Looks like it's a good fit then.