> -----Original Message----- > From: Peter Maydell [mailto:peter.mayd...@linaro.org] > Sent: Tuesday, February 06, 2018 10:36 PM > To: Gonglei (Arei) > Cc: QEMU Developers; Paolo Bonzini; Huangweidong (C) > Subject: Re: [PATCH v2] rtc: placing RTC memory region outside BQL > > On 6 February 2018 at 14:07, Gonglei <arei.gong...@huawei.com> wrote: > > As windows guest use rtc as the clock source device, > > and access rtc frequently. Let's move the rtc memory > > region outside BQL to decrease overhead for windows guests. > > Meanwhile, adding a new lock to avoid different vCPUs > > access the RTC together. > > > > $ cat strace_c.sh > > strace -tt -p $1 -c -o result_$1.log & > > sleep $2 > > pid=$(pidof strace) > > kill $pid > > cat result_$1.log > > > > Before appling this change: > > $ ./strace_c.sh 10528 30 > > % time seconds usecs/call calls errors syscall > > ------ ----------- ----------- --------- --------- ---------------- > > 93.87 0.119070 30 4000 ppoll > > 3.27 0.004148 2 2038 ioctl > > 2.66 0.003370 2 2014 futex > > 0.09 0.000113 1 106 read > > 0.09 0.000109 1 104 io_getevents > > 0.02 0.000029 1 30 poll > > 0.00 0.000000 0 1 write > > ------ ----------- ----------- --------- --------- ---------------- > > 100.00 0.126839 8293 total > > > > After appling the change: > > $ ./strace_c.sh 23829 30 > > % time seconds usecs/call calls errors syscall > > ------ ----------- ----------- --------- --------- ---------------- > > 92.86 0.067441 16 4094 ppoll > > 4.85 0.003522 2 2136 ioctl > > 1.17 0.000850 4 189 futex > > 0.54 0.000395 2 202 read > > 0.52 0.000379 2 202 io_getevents > > 0.05 0.000037 1 30 poll > > ------ ----------- ----------- --------- --------- ---------------- > > 100.00 0.072624 6853 total > > > > The futex call number decreases ~90.6% on an idle windows 7 guest. > > These are the same figures as from v1 -- it would be interesting > to check whether the additional locking that v2 adds has affected > the results. > Oh, yes. the futex number of v2 don't decline compared too much to v1 because it takes the BQL before raising the outbound IRQ line now.
Before applying v2: # ./strace_c.sh 8776 30 % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 78.01 0.164188 26 6436 ppoll 8.39 0.017650 5 3700 39 futex 7.68 0.016157 6 2758 ioctl 5.48 0.011530 3 4586 1113 read 0.30 0.000640 20 32 io_submit 0.15 0.000317 4 89 write ------ ----------- ----------- --------- --------- ---------------- 100.00 0.210482 17601 1152 total After applying v2: # ./strace_c.sh 15968 30 % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 78.28 0.171117 27 6272 ppoll 8.50 0.018571 5 3663 21 futex 7.76 0.016973 6 2732 ioctl 4.85 0.010597 3 4115 853 read 0.31 0.000672 11 63 io_submit 0.30 0.000659 4 180 write ------ ----------- ----------- --------- --------- ---------------- 100.00 0.218589 17025 874 total > Does the patch improve performance in a more interesting use > case than "the guest is just idle" ? > I think so, after all, the scope of the locking is reduced . Besides this, can we optimize the rtc timer to avoid to hold BQL by separate threads? > > +static void rtc_rasie_irq(RTCState *s) > > Typo: should be "raise". > Good catch. :) Thanks, -Gonglei