Before muxes were used, we used to refrain from reading past the
buffer's reserve. But with muxes which have their own buffer, this
rule was a bit forgotten, resulting in an extraneous read to be
performed just because the rx buffer cannot be entirely transferred
to the stream layer:
sendto(12, "GET /?s=16k HTTP/1.1\r\nhost: 127."..., 84, MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 84
recvfrom(12, "HTTP/1.1 200\r\nContent-length: 16"..., 16320, 0, NULL, NULL) = 16320
recvfrom(12, ".123456789.12345", 16, 0, NULL, NULL) = 16
recvfrom(12, "6789.123456789.12345678\n.1234567"..., 15244, 0, NULL, NULL) = 182
recvfrom(12, 0x1e5d5d6, 15062, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
Here the server sends 16kB of payload after a headers block, the mux reads
16320 into the ibuf, and the stream layer consumes 15360 from the first
h1_rcv_buf(), which leaves 960 into the buffer and releases a few indexes.
The buffer cannot be realigned due to these remaining data, and a subsequent
read is made on 16 bytes, then again on 182 bytes.
By avoiding to read too much on the first call, we can avoid needlessly
filling this buffer:
recvfrom(12, "HTTP/1.1 200\r\nContent-length: 16"..., 15360, 0, NULL, NULL) = 15360
recvfrom(12, "456789.123456789.123456789.12345"..., 16220, 0, NULL, NULL) = 1158
recvfrom(12, 0x1d52a3a, 15062, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
This is much more efficient and uses less RAM since the first buffer that
was emptied can now be released.
Note that a further improvement (tested) consists in reading even less
(typically 1kB) so that most of the data are transferred in zero-copy, and
are not read until process_stream() is scheduled. This patch doesn't do that
for now so that it can be backported without any obscure impact.
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