Re: [Qemu-devel] [PATCH 07/15] dataplane: use object pool to speed up allocation for virtio blk request

[Paolo Bonzini <pbonzini@redhat.com>]

Il 31/07/2014 05:22, Ming Lei ha scritto:
>> >
>> > The problem is that g_slice here is not using the slab-style allocator
>> > because the object is larger than roughly 500 bytes.  One solution would
>> > be to make virtqueue_pop/vring_pop allocate a VirtQueueElement of the
>> > right size (and virtqueue_push/vring_push free it), as mentioned in the
>> > review of patch 8.
> Unluckily both iovec and addr array can't be fitted into 500 bytes, :-(
> Not mention all users of VirtQueueElement need to be changed too,
> I hate to make that work involved in this patchset, :-)

Well, the point of dataplane was not just to get maximum iops.  It was
also to provide guidance in the work necessary to improve the code and
get maximum iops without special-casing everything.  This can be a lot
of work indeed.

>> >
>> > However, I now remembered that VirtQueueElement is a mess because it's
>> > serialized directly into the migration state. :(  So you basically
>> > cannot change it without mucking with migration.  Please leave out patch
>> > 8 for now.
> save_device code serializes elem in this way:
> 
>  qemu_put_buffer(f, (unsigned char *)&req->elem,
>                         sizeof(VirtQueueElement));
> 
> so I am wondering why this patch may break migration.

Because you change the on-wire format and break migration from 2.1 to
2.2.  Sorry, I wasn't clear enough.

Paolo

> And in my test live migration can survive with the patch.
>