On 2011-02-23 12:08, Edgar E. Iglesias wrote: > On Wed, Feb 23, 2011 at 11:25:54AM +0100, Paolo Bonzini wrote: >> On 02/23/2011 11:18 AM, Edgar E. Iglesias wrote: >>> Sorry, I don't know the code well enough to give any sensible feedback >>> on patch 2 - 4. I did test them with some of my guests and things seem >>> to be OK with them but quite a bit slower. >>> I saw around 10 - 20% slowdown with a cris guest and -icount 10. >>> >>> The slow down might be related to the issue with super slow icount together >>> with iothread (adressed by Marcelos iothread timeout patch). >> >> No, this supersedes Marcelo's patch. 10-20% doesn't seem comparable to >> "looks like it deadlocked" anyway. Also, Jan has ideas on how to remove >> the synchronization overhead in the main loop for TCG+iothread. > > I see. I tried booting two of my MIPS and CRIS linux guests with iothread > and -icount 4. Without your patch, the boot crawls super slow. Your patch > gives a huge improvement. This was the "deadlock" scenario which I > mentioned in previous emails. > > Just to clarify the previous test where I saw slowdown with your patch: > A CRIS setup that has a CRIS and basically only two peripherals, > a timer block and a device (X) that computes stuff but delays the results > with a virtual timer. The guest CPU is 99% of the time just > busy-waiting for device X to get ready. > > This latter test runs in 3.7s with icount 4 and without iothread, > with or without your patch. > > With icount 4 and iothread it runs in ~1m5s without your patch and > ~1m20s with your patch. That was the 20% slowdown I mentioned earlier. > > Don't know if that info helps...
You should try to trace the event flow in qemu, either via strace, via the built-in tracer (which likely requires a bit more tracepoints), or via a system-level tracer (ftrace / kernelshark). Did my patches contribute a bit to overhead reduction? They specifically target the costly vcpu/iothread switches in TCG mode (caused by TCGs excessive lock-holding times). Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux
