Re: [Qemu-devel] [PATCH v3 9/9] coalesce adjacent iovecs

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 21, 2013 at 06:40:24PM +0200, Orit Wasserman wrote:
> On 03/21/2013 06:29 PM, Michael S. Tsirkin wrote:
> > On Thu, Mar 21, 2013 at 06:27:42PM +0200, Orit Wasserman wrote:
> >> On 03/21/2013 06:16 PM, Michael S. Tsirkin wrote:
> >>> On Thu, Mar 21, 2013 at 06:05:40PM +0200, Orit Wasserman wrote:
> >>>> This way we send one big buffer instead of many small ones
> >>>>
> >>>> Signed-off-by: Orit Wasserman <owasserm@redhat.com>
> >>>
> >>> Why does this happen BTW?
> >>
> >> It happens in the last phase when we send the device state that consists of a lot
> >> bytes and int field that are written using qemu_put_byte/be16/...
> >>
> > 
> > Confused  I thought device_state does not use _nocopy?
> > My idea of using vmsplice relies exactly on this:
> > we can not splice device state ...
> 
> qemu_put_buffer calls qemu_put_buffer_no_copy (you know code reuse)
> So I guess we will need to add some flag if we want to use vmsplice or not
> or split no_copy into external and internal. It won't be a big change

Fair enough. Have time to look into this? Data copies is like 15% CPU,
more with swap ...

> > 
> >>>
> >>>> ---
> >>>>  savevm.c | 21 +++++++++++++++++----
> >>>>  1 file changed, 17 insertions(+), 4 deletions(-)
> >>>>
> >>>> diff --git a/savevm.c b/savevm.c
> >>>> index 50e8fb2..13a533b 100644
> >>>> --- a/savevm.c
> >>>> +++ b/savevm.c
> >>>> @@ -634,8 +634,14 @@ void qemu_put_buffer_no_copy(QEMUFile *f, const uint8_t *buf, int size)
> >>>>          abort();
> >>>>      }
> >>>>  
> >>>> -    f->iov[f->iovcnt].iov_base = (uint8_t *)buf;
> >>>> -    f->iov[f->iovcnt++].iov_len = size;
> >>>> +    /* check for adjoint buffer and colace them */
> >>>> +    if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
> >>>> +        f->iov[f->iovcnt - 1].iov_len) {
> >>>> +        f->iov[f->iovcnt - 1].iov_len += size;
> >>>> +    } else {
> >>>> +        f->iov[f->iovcnt].iov_base = (uint8_t *)buf;
> >>>> +        f->iov[f->iovcnt++].iov_len = size;
> >>>> +    }
> >>>>  
> >>>>      f->is_write = 1;
> >>>>      f->bytes_xfer += size;
> >>>> @@ -690,8 +696,15 @@ void qemu_put_byte(QEMUFile *f, int v)
> >>>>      f->buf[f->buf_index++] = v;
> >>>>      f->is_write = 1;
> >>>>      f->bytes_xfer += 1;
> >>>> -    f->iov[f->iovcnt].iov_base = f->buf + (f->buf_index - 1);
> >>>> -    f->iov[f->iovcnt++].iov_len = 1;
> >>>> +
> >>>> +    /* check for adjoint buffer and colace them */
> >>>> +    if (f->iovcnt > 0 && f->buf + (f->buf_index - 1)  ==
> >>>> +        f->iov[f->iovcnt - 1].iov_base + f->iov[f->iovcnt - 1].iov_len) {
> >>>> +        f->iov[f->iovcnt - 1].iov_len += 1;
> >>>> +    } else {
> >>>> +        f->iov[f->iovcnt].iov_base = f->buf + (f->buf_index - 1);
> >>>> +        f->iov[f->iovcnt++].iov_len = 1;
> >>>> +    }
> >>>>  
> >>>>      if (f->buf_index >= IO_BUF_SIZE || f->iovcnt >= MAX_IOV_SIZE) {
> >>>>          qemu_fflush(f);
> >>>> -- 
> >>>> 1.7.11.7