Re: [Qemu-devel] [PATCH 1.2 0/7] Manual writethrough cache and cache mode toggle

[Kevin Wolf]

Am 22.05.2012 18:17, schrieb Paolo Bonzini:
> This is an alternative implementation of writethrough caching.  By always
> opening protocols in writethrough mode and doing flushes manually after
> every write, it achieves two results: 1) it makes flipping the cache mode
> extremely easy; 2) it lets formats control flushes during metadata updates
> even in writethrough mode, which makes the updates more efficient; 3)
> it makes cache=writethrough automatically flush metadata without needing
> extra work in the formats.
> 
> In practice, the performance result is a wash.  I measured "make -j3
> vmlinux" on a 2-core guest/4-core host, with 2GB memory in the guest
> and 8GB in the host.
> 
> Performance was measured starting qemu-kvm with an empty qcow2 image,
> a virtio disk and cache=writethrough (F16 installation + exploded kernel
> tarball in the backing file), and the results are as follows:
> 
>     without patch:
>     real    9m22.416s user    12m8.955s sys     3m46.331s
>     real    9m21.602s user    12m20.124s sys     3m51.126s
>     real    9m22.258s user    12m14.900s sys     3m48.595s
> 
>     with patch:
>     real    9m17.617s user    12m16.837s sys     3m48.637s
>     real    9m18.926s user    12m20.083s sys     3m50.458s
>     real    9m15.751s user    12m41.944s sys     3m56.447s
> 
> Unlike the RFC, this is tested exactly with these seven patches.  The RFC
> was tested with follow-up cleanups that hid the problem in patch 3.
> 
> v1->v2: only patch 3 changed, was completely backwards in v1

Are you going to send a v3 for patches 3 and possibly 5/6?

Kevin