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

[Orit Wasserman <owasserm@redhat.com>]

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

> 
>>>
>>>> ---
>>>>  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