Re: [Qemu-devel] [PATCH v2 4/4] libqos/ahci: Swap memread/write with bufread/write

[John Snow]

On 05/01/2015 04:48 PM, Paolo Bonzini wrote:
> On 01/05/2015 21:55, John Snow wrote:
> > Where it makes sense, use the new faster primitives.
> > For generally small reads/writes such as for the PRDT
> > and FIS packets, stick with the more wasteful but
> > easier to debug memread/memwrite.
> >
> > For ahci-test;
> > With this patch:
> > real    0m4.802s
> > user    0m3.506s
> > sys     0m2.393s
> >
> > Without this series:
> > real    0m14.171s
> > user    0m12.072s
> > sys     0m12.527s
>
> The overhead of memread is 2, the overhead of base64 is 1.33, also
> base64 should have a larger cost of computing each byte.  It doesn't add up.
>
> Could it be simply that calling qtest_send (and hence
> vsnprintf+qemu_chr_fe_write_all, neither of which are speed demons) once
> per byte is hideously inefficient? :)
>
> Paolo

I wrote a loop to batch the ascii-hex conversion instead of letting 
printf do it; then ran some more very, very scientific tests:

memset alone:
real    0m10.888s
user    0m9.303s
sys     0m9.146s

send-batching:
real    0m6.541s
user    0m5.027s
sys     0m4.941s

memset+batching+b64:
real    0m3.675s
user    0m2.582s
sys     0m1.718s

So it still seems as if the b64 batching is a strict improvement 
speed-wise. I'll send the non-b64 batching patch separately later, 
unless you have thoughts otherwise.

--js