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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v2 4/4] libqos/ahci: Swap memread/write with bufread/write
Date: Fri, 01 May 2015 22:48:08 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

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? :)


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