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

[Paolo Bonzini <pbonzini@redhat.com>]

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