Re: [PATCH v4 09/11] hw/9pfs/9p-synth: avoid n-square issue in synth_readdir()

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Donnerstag, 23. Januar 2020 12:13:51 CET Greg Kurz wrote:
> Honestly it doesn't seem to change anything significant for me.
> Mean time calculated over 100 runs:
> 
> Without this patch:
> 
> [greg@bahia qemu-9p]$ (cd .mbuild-$(stg branch)/obj ; export
> QTEST_QEMU_BINARY='x86_64-softmmu/qemu-system-x86_64'; make all
> tests/qtest/qos-test && for i in {1..100}; do tests/qtest/qos-test -p
> $(tests/qtest/qos-test -l | grep readdir/basic); done) |& awk '/IMPORTANT/
> { print $10 }' | sed -e 's/s//' -e 's/^/n+=1;x+=/;$ascale=6;x/n' | bc
> .055654
> 
> With this patch:
> 
> [greg@bahia qemu-9p]$ (cd .mbuild-$(stg branch)/obj ; export
> QTEST_QEMU_BINARY='x86_64-softmmu/qemu-system-x86_64'; make all
> tests/qtest/qos-test && for i in {1..100}; do tests/qtest/qos-test -p
> $(tests/qtest/qos-test -l | grep readdir/basic); done) |& awk '/IMPORTANT/
> { print $10 }' | sed -e 's/s//' -e 's/^/n+=1;x+=/;$ascale=6;x/n' | bc
> .058786

:))) Mhmm, that's because you have run this test WITHOUT the actual readdir 
optimization patch. In this scenario the readdir latency issue is so bad, that 
the driver's n-square issue does not even matter, so same here:

Unoptimized readdir, no n-squre correction hack:
Time client spent for waiting for reply from server: 0.082831s [MOST 
IMPORTANT]

Unoptimized readdir, with n-squre correction hack:
Time client spent for waiting for reply from server: 0.082539s [MOST 
IMPORTANT]

BUT, now look at this *with* readdir optimization applied:

Optimized readdir, no n-square correction hack:
Time 9p server spent on synth_readdir() I/O only (synth driver): 0.021304s
Time 9p server spent on entire T_readdir request: 0.022033s [IMPORTANT]
Time client spent for waiting for reply from server: 0.022408s [MOST 
IMPORTANT]

Optimized readdir, with n-square correction hack:
Time 9p server spent on synth_readdir() I/O only (synth driver): 0.001576s
Time 9p server spent on entire T_readdir request: 0.002244s [IMPORTANT]
Time client spent for waiting for reply from server: 0.002566s [MOST 
IMPORTANT]

Got it? :)

I had good reasons for printing out the time spent on raw driver only.

Best regards,
Christian Schoenebeck