Re: [Qemu-devel] [PATCH v2 0/3] virtio: proposal to optimize accesses to VQs

[Paolo Bonzini <pbonzini@redhat.com>]

On 15/12/2015 23:33, Vincenzo Maffione wrote:
> This patch slightly rewrites the code to reduce the number of accesses, since
> many of them seems unnecessary to me. After this reduction, the bottleneck
> jumps from 1 Mpps to 2 Mpps.

Very nice.  Did you get new numbers with the rebase?  That would help
measuring the effect of removing variable-sized memcpy (I'll post the
patches for this shortly; they're entirely in memory.h/exec.c so they're
not virtio-specific).  A rough measurement from "perf" says they're
worth about 5%.

Related to this, patch 3 introduces a variable-sized memcpy, because it
switches from 2 virtio_stl_phys to 1 address_space_write.  I'm curious
if the effect of this individual patch is positive, negative or neutral.
 On the other hand, patches 1 and 2 are clear wins.

Paolo

> Patch is not complete (e.g. it still does not properly manage endianess, it is
> not clean, etc.). I just wanted to ask if you think the idea makes sense, and
> a proper patch in this direction would be accepted.