On Mon, 9 Jan 2023, Dmitry Karpov via curl-library wrote:
In H3, Curl's internal buffering also controls QUIC internal buffering,
While this might be the case right now for one/some/all of the backends, I don't think this is something that is carved in stone. curl uses the buffer in one way, but there's nothing that prevents the QUIC stacks to use their own buffers much in the same way TCP does in the kernel.
In other words, to get good download performance with H3, client will have to increase curl handle read buffer size and be ready to handle much larger data blocks in less frequent "write callback" calls compared to H2/H3. This may force client apps to have different handlings of H3 and H1/H2 caused by changed "write callback" frequency and increased data size.
Sure, but none of that is carved in stone either and may vary depending on what backends you use or even what versions of the involved components. Also, just using different servers and network speeds can make them vary quite a lot as well.
The existing separation between Curl handle's internal buffering and TCP transport layer buffering in H1/H2 made me think about a special "QUIC" option controlling QUIC buffering and flow control, while keeping the "read buffer size" option separate from that - so as in H1/H2 it will only control the frequency and size of data blocks used in the "write callback" calls.
And while I think that your change is very helpful to have big download buffers in H3, we may still probably need to think about if we want to use CURLOPT_BUFFERSIZE option to control H3/QUIC transport buffering, considering the side effects I mentioned above.
This sounds like you're talking about something more than just a buffer size option though; a separate buffering system somehow.
But for sure, I'm open for discussing, testing and experimenting with different approaches if such new ways can give us better performance or other benefits that users will appreciate!
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