Re: Fio regression caused by f9fc8932b11f3bcf2a2626f567cb6fdd36a33a94

[Paolo Bonzini <pbonzini@redhat.com>]

On 5/6/22 06:30, Lukáš Doktor wrote:
> Hello all,
> 
> thank you for the responses, I ran 3 runs per each commit using 5 iteration of fio-nbd using
> 
> f9fc8932b11f3bcf2a2626f567cb6fdd36a33a94
> f9fc8932b11f3bcf2a2626f567cb6fdd36a33a94 + Stefan's commit
> d7482ffe9756919531307330fd1c6dbec66e8c32

Ok, there's another simple change that can be made to bring performance
back to 6.2 levels, actually a bit better.  I'll post patches soon.
Here are 4 fio runs:

6.2:
    iops        : min=58051, max=62260, avg=60282.57, stdev=1081.18, samples=30
     clat percentiles (usec):   1.00th=[  490],   99.99th=[  775]
    iops        : min=59401, max=61290, avg=60651.27, stdev=468.24, samples=30
     clat percentiles (usec):   1.00th=[  490],   99.99th=[  717]
    iops        : min=59583, max=60816, avg=60353.43, stdev=282.69, samples=30
     clat percentiles (usec):   1.00th=[  490],   99.99th=[  701]
    iops        : min=58099, max=60713, avg=59739.53, stdev=755.49, samples=30
     clat percentiles (usec):   1.00th=[  494],   99.99th=[  717]


patched:
    iops        : min=60616, max=62522, avg=61654.37, stdev=555.67, samples=30
     clat percentiles (usec):   1.00th=[  474],   99.99th=[ 1303]
    iops        : min=61841, max=63600, avg=62878.47, stdev=442.40, samples=30
     clat percentiles (usec):   1.00th=[  465],   99.99th=[  685]
    iops        : min=62976, max=63910, avg=63531.60, stdev=261.05, samples=30
     clat percentiles (usec):   1.00th=[  461],   99.99th=[  693]
    iops        : min=60803, max=63623, avg=62653.37, stdev=808.76, samples=30
     clat percentiles (usec):   1.00th=[  465],   99.99th=[  685]


I also played a bit with direct wakeup of threads using a QemuEvent per thread.
Peak performance is higher (low percentiles are better) but the problem is that
it doesn't necessarily pick the most effective thread for wakeup resulting in
oscillations:

    iops        : min=60971, max=65726, avg=63771.93, stdev=1381.06, samples=30
     clat percentiles (usec): 1.00th=[  457],  99.99th=[  685]
    iops        : min=57537, max=64914, avg=63694.37, stdev=1809.40, samples=30
     clat percentiles (usec): 1.00th=[  461],  99.99th=[  693]
    iops        : min=58175, max=64711, avg=61277.80, stdev=2216.05, samples=30
     clat percentiles (usec): 1.00th=[  465],  99.99th=[  685]
    iops        : min=56349, max=63938, avg=58442.33, stdev=2012.54, samples=30
     clat percentiles (usec): 1.00th=[  469],  99.99th=[  668]

I'll go for the simple one.

Paolo