[xcb] XCB polling and single-threadedness

[Bart Massey]

When I call XCBPollEvent() I want to be sure I get an
event if one is available right now.  Thus, XCB should
keep reading until either it has read the first complete
event off the wire or until the wire shows empty.

This begs the obvious question.  For a number of purposes
(e.g. exposure compression) it may be desirable to read a
bunch of events at once.  Is there some efficiency reason to
provide XCBPollEvents() which can return multiple events?  I
don't think so, but I might be missing something...

	Bart

In message <ye8it3qh4fg.fsf@horse09.daimi.au.dk> you wrote:
> Jamey Sharp <jamey@sharp.ath.cx> writes:
> 
> > > The patch below makes XCBFillBuffer empty the file descriptor instead
> > > of calling read() just once.
> > 
> > Is there any reason to believe that this will make any difference? I
> > figure that if there's more data readable after the first read, then
> > XCB's buffer was filled on the first call. If that's true, the second
> > read will be an expensive no-op under any circumstances.
> 
> I don't know if it will make any real difference, but in some
> unrelated networking code I have someting like this:
> 
>     #define BUF_SIZE 8192
> 
>     while (TRUE)
>     {
>         unsigned char msg [BUF_SIZE];
>         ssize_t len;
>         
>         len = recv (connection->fd, msg, BUF_SIZE)
>         if (len < 0)
>         {
>             if (errno != EAGAIN)
>                  /* handle error */
>             break;
>         }
>         
>         if (len == 0)
>         {
>             /* handle close */
>             break;
>         }
>         
>         /* queue read handler */
>     }
> 
> and while most second recv()s are noops, it is not that uncommon for
> strace to report things like:
> 
> [...]
> 20:57:10.178201 poll([{fd=5, events=POLLIN, revents=POLLIN}], 1, 49) = 1
> 20:57:10.190586 recv(5, "HTTP/1.1 200 OK\r\nDate: Sun, 07 J"..., 8192, 0) = 1368
> 20:57:10.190742 recv(5, "Accept-Ranges: bytes\r\nContent-Le"..., 8192, 0) = 1368
> 20:57:10.190846 recv(5, "\234\204J\234\214s\245s\0\245s\10\245\224{\245\245\245"..., 8192, 0) = 1368
> 20:57:10.190988 recv(5, "\315\227\"\244\256w\r\200\347\235\216,\245\200\236\0>\24"..., 8192, 0) = 1106
> 20:57:10.191072 recv(5, 0xbfffc2b8, 8192, 0) = -1 EAGAIN (Resource temporarily unavailable)
> 20:57:10.192198 poll([{fd=5, events=POLLIN, revents=POLLIN}], 1, 35) = 1
> [...]
> 
> ie, four recv()s in a row, all of which are much smaller than
> BUF_SIZE.
> 
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