On Wed, Mar 16, 2011 at 2:20 PM, Venkateswararao Jujjuri (JV) <jv...@linux.vnet.ibm.com> wrote: > 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.
Sounds fair. Stefan
