Re: [Qemu-devel] [PATCH v2] rtc: placing RTC memory region outside BQL

[Peter Maydell]

On 6 February 2018 at 14:07, Gonglei <address@hidden> 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.

Does the patch improve performance in a more interesting use
case than "the guest is just idle" ?

> +static void rtc_rasie_irq(RTCState *s)

Typo: should be "raise".

thanks
-- PMM