Re: [PATCH v2 1/5] virtio: add functions for piecewise addition ofbuffers

[Wanlong Gao]

On 01/02/2013 01:03 PM, Rusty Russell wrote:
> Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
>> The virtqueue_add_buf function has two limitations:
>>
>> 1) it requires the caller to provide all the buffers in a single call;
>>
>> 2) it does not support chained scatterlists: the buffers must be
>> provided as an array of struct scatterlist;
> 
> Chained scatterlists are a horrible interface, but that doesn't mean we
> shouldn't support them if there's a need.
> 
> I think I once even had a patch which passed two chained sgs, rather
> than a combo sg and two length numbers.  It's very old, but I've pasted
> it below.
> 
> Duplicating the implementation by having another interface is pretty
> nasty; I think I'd prefer the chained scatterlists, if that's optimal
> for you.

I rebased against virtio-next and use it in virtio-scsi, and tested it with 4 targets
virtio-scsi devices and host cpu idle=poll. Saw a little performance regression here.

General:
Run status group 0 (all jobs):
   READ: io=34675MB, aggrb=248257KB/s, minb=248257KB/s, maxb=248257KB/s, mint=143025msec, maxt=143025msec
  WRITE: io=34625MB, aggrb=247902KB/s, minb=247902KB/s, maxb=247902KB/s, mint=143025msec, maxt=143025msec

Chained:
Run status group 0 (all jobs):
   READ: io=34863MB, aggrb=242320KB/s, minb=242320KB/s, maxb=242320KB/s, mint=147325msec, maxt=147325msec
  WRITE: io=34437MB, aggrb=239357KB/s, minb=239357KB/s, maxb=239357KB/s, mint=147325msec, maxt=147325msec

Thanks,
Wanlong Gao