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

[Paolo Bonzini]

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.

Paolo