[Spice-devel] [spice-server PATCH] RFC: stream-device: do not reallocate dev->msg for each frame

[Frediano Ziglio]

> 
> On 07/24/2018 01:25 PM, Frediano Ziglio wrote:
> >>
> >> In the past dev->msg was only allocated for cursor images, and
> >> after use dev->msg size was reduced.
> >>
> >> Now, since commit dcc3f995d9f55, dev->msg is used for every frame.
> >> There is little point of calling reallocate with a large dev->msg size
> >> and then reallocate with a small size, for every frame.
> >>
> >> This patch keeps only the "enlarge buffer when needed" part.
> >>
> >> The drawback of this approach is that if a big buffer was used
> >> it is kept big (at least, in the near future), even if
> >> current frames are smaller.
> >>
> >> Signed-off-by: Uri Lublin <uril at redhat.com>
> > 
> > Why RFC?
> 
> So people can comment :)
> 

Honestly in this ML RFC usually is close to /dev/null...

> I think the benefit (not re-allocating for each frame) is more valuable
> than the drawback (keeping a large buffer).
> 

Optimizations without numbers are bets, do you have numbers?

> > 
> > Looking at current code looks like for similar usage we use a static
> > "small" (some KB) buffer as most messages are small and do dynamic
> > alloc/free for every other message.
> 
> The static buffer is only used upon an error.

I'm talking about message allocation in the channels, like common_alloc_recv_buf
(server/common-graphics-channel.c), is a quite common pattern in SPICE server.

> All messages are read into dev->msg->buf.
> 
> Also once streaming starts there are a lot of consecutive
> "DATA" messages (frames).
> 

I know. One way I was thinking is adding a timer (let say 1 minute) on first
reallocation/increase and once timer trigger shrink the buffer, maximum
will resize every 1 minute.

Still... all thinking without numbers!
A simple set of message sizes and a reproducer just doing g_malloc/whatever
would be enough.

> Thanks,
>      Uri.
> 
> 
> 
> > 
> >> ---
> >>   server/red-stream-device.c | 14 --------------
> >>   1 file changed, 14 deletions(-)
> >>
> >> diff --git a/server/red-stream-device.c b/server/red-stream-device.c
> >> index d293dc1cc..eda6e6690 100644
> >> --- a/server/red-stream-device.c
> >> +++ b/server/red-stream-device.c
> >> @@ -175,14 +175,6 @@ stream_device_partial_read(StreamDevice *dev,
> >> SpiceCharDeviceInstance *sin)
> >>        * the next message */
> >>       if (handled) {
> >>           dev->hdr_pos = 0;
> >> -
> >> -        // Reallocate message buffer to the minimum.
> >> -        // Currently the only message that requires resizing is the
> >> cursor
> >> shape,
> >> -        // which is not expected to be sent so often.
> >> -        if (dev->msg_len > sizeof(*dev->msg)) {
> >> -            dev->msg = g_realloc(dev->msg, sizeof(*dev->msg));
> >> -            dev->msg_len = sizeof(*dev->msg);
> >> -        }
> >>       }
> >>   
> >>       if (handled || dev->has_error) {
> >> @@ -314,12 +306,6 @@ handle_msg_data(StreamDevice *dev,
> >> SpiceCharDeviceInstance *sin)
> >>       }
> >>   
> >>       /* make sure we have a large enough buffer for the whole frame */
> >> -    /* ---
> >> -     * TODO: Now that we copy partial data into the buffer, for each
> >> frame
> >> -     * the buffer is allocated and freed (look for g_realloc in
> >> -     * stream_device_partial_read).
> >> -     * Probably better to just keep the larger buffer.
> >> -     */
> >>       if (dev->msg_pos == 0) {
> >>           dev->frame_mmtime = reds_get_mm_time();
> >>           if (dev->msg_len < dev->hdr.size) {
> >