[Libguestfs] [libnbd PATCH 2/1] states: Avoid wasted send() when REPLY interrupts request

When we are blocked waiting for POLLOUT during a request, and happen to receive notice of POLLIN instead, we know that the work done in response to POLLIN will be non-blocking (it returns to %.READY as soon as it would block, which in turn jumps right back into ISSUE_COMMAND because we have a pending request not fully sent yet). Since the jaunt through REPLY was non-blocking, it is unlikely that the POLLOUT situation has changed in the meantime, so if we use SET_NEXT_STATE() to step back into SEND_REQUEST, our recv() call will likely fail with EAGAIN, once again blocking us until our next POLLOUT. Although the wasted syscall is not on the hot-path (after all, we can't progress until data arrives from the server), it's slightly cleaner if we instead declare that we are already blocked. I tested with: $ nbdkit -U - null 16M --run 'examples/threaded-reads-and-writes $unixsocket' There was no real noticeable difference in timing, but I did observe that pre-patch, the run encountered 168825 pre-emptions and 136976 send() EAGAIN failures (81%), while post-patch the run encountered 166066 pre-emptions and 552 EAGAIN failures (0.3%). --- generator/states-issue-command.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/generator/states-issue-command.c b/generator/states-issue-command.c index 821b28a..f746f80 100644 --- a/generator/states-issue-command.c +++ b/generator/states-issue-command.c @@ -25,12 +25,17 @@ assert (h->cmds_to_issue != NULL); cmd = h->cmds_to_issue; - /* Were we interrupted by reading a reply to an earlier command? */ + /* Were we interrupted by reading a reply to an earlier command? If + * so, we can only get back here after a non-blocking jaunt through + * the REPLY engine, which means we are unlikely to be unblocked for + * writes yet; we want to advance back to the correct state but + * without trying a send_from_wbuf that will likely return 1. + */ if (h->wlen) { if (h->in_write_payload) - SET_NEXT_STATE(%SEND_WRITE_PAYLOAD); + *next_state = %SEND_WRITE_PAYLOAD; else - SET_NEXT_STATE(%SEND_REQUEST); + *next_state = %SEND_REQUEST; return 0; } -- 2.20.1