tree d3101509e8995e5224dcb11ea57511e02b1528ad
parent 13cd54c08b8ae78a619349a5251ac0e6001d9290
author Willy Tarreau <w@1wt.eu> 1592463527 +0200
committer Willy Tarreau <w@1wt.eu> 1592569306 +0200
encoding latin1

MEDIUM: fd: add experimental support for edge-triggered polling

Some of the recent optimizations around the polling to save a few
epoll_ctl() calls have shown that they could also cause some trouble.
However, over time our code base has become totally asynchronous with
I/Os always attempted from the upper layers and only retried at the
bottom, making it look like we're getting closer to EPOLLET support.

There are showstoppers there such as the listeners which cannot support
this. But given that most of the epoll_ctl() dance comes from the
connections, we can try to enable edge-triggered polling on connections.

What this patch does is to add a new global tunable "tune.fd.edge-triggered",
that makes fd_insert() automatically set an et_possible bit on the fd if
the I/O callback is conn_fd_handler. When the epoll code sees an update
for such an FD, it immediately registers it in both directions the first
time and doesn't update it anymore.

On a few tests it proved quite useful with a 14% request rate increase in
a H2->H1 scenario, reducing the epoll_ctl() calls from 2 per request to
2 per connection.

The option is obviously disabled by default as bugs are still expected,
particularly around the subscribe() code where it is possible that some
layers do not always re-attempt reading data after being woken up.