[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?
> 

Got this systemtap script

probe process("/usr/lib64/libspice-server.so.1.12.4").statement("*@red-stream-device.c:136")
{
        printf("size %d\n", $dev->hdr->size);
}

(line 136 is 

            dev->msg_pos = 0;

in

        if (dev->hdr_pos >= sizeof(dev->hdr)) {
            dev->hdr.type = GUINT16_FROM_LE(dev->hdr.type);
            dev->hdr.size = GUINT32_FROM_LE(dev->hdr.size);
            dev->msg_pos = 0;
        }
)

numbers were usually quite small (mostly less than 2kb). But was not doing much honestly

> > > 
> > > 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) {
> > > 
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