Re: [PATCH 3/5] tap: Don't use EPOLLET on Qemu sockets

On Fri, 26 Jul 2024 10:00:56 +0200 Stefano Brivio wrote:

On Fri, 26 Jul 2024 17:20:29 +1000 David Gibson wrote:

...

Currently we set EPOLLET (edge trigger) on the epoll flags for the connected Qemu Unix socket. It's not clear that there's a reason for doing this: for TCP sockets we need to use EPOLLET, because we leave data in the socket buffers for our flow control handling. That consideration doesn't apply to the way we handle the qemu socket however.

It significantly decreases epoll_wait() overhead on sustained data transfers, because we can read multiple TAP_BUF_SIZE buffers at a time instead of just one.

I can check that now again with current QEMU and kernel versions, plus several fundamental changes in buffer handling, but I don't see a real reason why this shouldn't have changed meanwhile.

Surprisingly, this doesn't affect throughput at all on my setup, no matter the packet size. I didn't check with perf(1), though, and we probably should give all the recent substantial changes a pass with it (it's been a while since I last checked where overhead typically is...). -- Stefano