On 3/16/2011 6:10 AM, Anthony Liguori wrote: > On 03/16/2011 04:20 AM, Stefan Hajnoczi wrote: >> On Tue, Mar 15, 2011 at 1:13 PM, Anthony Liguori<aligu...@us.ibm.com> wrote: >>> Why even bothering signaling for completion with the virtio-9p threadpool? >>> >>> There's no sane guest that's going to poll on virtio-9p completion with >>> interrupts disabled and no timer. Once we enable the I/O thread by default, >>> it won't even be necessary for the paio layer. >> It's not just about preventing livelock under extreme cases. If you >> omit the signal then !CONFIG_IOTHREAD builds will see increased >> latency because virtfs request completion will piggy-back on other >> events that *do* interrupt the vcpu. > > But realistically, the timer is firing at a high enough frequency that I doubt > you'd even observe the latency. > > There's an easy solution here, just do some sniff testing to see if you can > tell > the difference. I bet you can't. > >> I'm no fan of !CONFIG_IOTHREAD >> but skipping signals is currently bad practice and will continue to be >> until CONFIG_IOTHREAD is removed entirely. >> >> The proper solution would be a thin abstraction for thread-safe >> notification that compiles out signals when CONFIG_IOTHREAD is used. >> Then we have one place in QEMU that does signalling correctly and we >> can optimize it or remove CONFIG_IOTHREAD completely without having >> the burden of duplicating this code in several places. > > We have probably 5 different ways to wake up a CPU. I don't think we should > add > a new one just for this. > > !CONFIG_IOTHREAD needs to go away in the very near future. I'd rather focus > on > that.
If that is the case, how about making VirtFS dependent on CONFIG_IOTHREAD during the configuration? This can help even if !CONFIG_IOTHREAD lingers around for some more time and would avoid any of the Stefan's concerns. - JV > > Regards, > > Anthony Liguori > >> Stefan >
