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pipe: Increase the writer-wakeup threshold to reduce context-switch count

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Increase the threshold at which the reader sends a wake event to the
writers in the queue such that the queue must be half empty before the wake
is issued rather than the wake being issued when just a single slot
available.

This reduces the number of context switches in the tests significantly,
without altering the amount of work achieved.  With my pipe-bench program,
there's a 20% reduction versus an unpatched kernel.

Suggested-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: David Howells <dhowells@redhat.com>
This commit is contained in:
David Howells 2019-10-31 15:59:24 +00:00
parent 8df441294d
commit cefa80ced5
1 changed files with 4 additions and 2 deletions
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6
fs/pipe.c
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@ -324,16 +324,18 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
}
if (!buf->len) {
bool wake;
pipe_buf_release(pipe, buf);
spin_lock_irq(&pipe->wait.lock);
tail++;
pipe->tail = tail;
do_wakeup = 1;
if (head - (tail - 1) == pipe->max_usage)
wake = head - (tail - 1) == pipe->max_usage / 2;
if (wake)
wake_up_interruptible_sync_poll_locked(
&pipe->wait, EPOLLOUT | EPOLLWRNORM);
spin_unlock_irq(&pipe->wait.lock);
if (head - (tail - 1) == pipe->max_usage)
if (wake)
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}
total_len -= chars;