Commit cefa80ce authored Oct 31, 2019 by David Howells

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



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>

parent 8df44129

fs/pipe.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -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;

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