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Date: Tue, 13 Dec 2011 15:44:45 -0600
From: Dave Kleikamp <dave.kleikamp@oracle.com>
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To: linux-aio@kvack.org
CC: linux-kernel@vger.kernel.org, Chris Mason <chris.mason@oracle.com>,
        Jens Axboe <axboe@kernel.dk>, Andi Kleen <ak@linux.intel.com>,
        Jeff Moyer <jmoyer@redhat.com>
Subject: [PATCH] AIO: Don't plug the I/O queue in do_io_submit()
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Asynchronous I/O latency to a solid-state disk greatly increased
between the 2.6.32 and 3.0 kernels. By removing the plug from
do_io_submit(), we observed a 34% improvement in the I/O latency.

Unfortunately, at this level, we don't know if the request is to
a rotating disk or not.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: linux-aio@kvack.org
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jeff Moyer <jmoyer@redhat.com>

diff --git a/fs/aio.c b/fs/aio.c
index 78c514c..d131a2c 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1696,7 +1696,6 @@ long do_io_submit(aio_context_t ctx_id, long nr,
 	struct kioctx *ctx;
 	long ret = 0;
 	int i = 0;
-	struct blk_plug plug;
 	struct kiocb_batch batch;
 
 	if (unlikely(nr < 0))
@@ -1716,8 +1715,6 @@ long do_io_submit(aio_context_t ctx_id, long nr,
 
 	kiocb_batch_init(&batch, nr);
 
-	blk_start_plug(&plug);
-
 	/*
 	 * AKPM: should this return a partial result if some of the IOs were
 	 * successfully submitted?
@@ -1740,7 +1737,6 @@ long do_io_submit(aio_context_t ctx_id, long nr,
 		if (ret)
 			break;
 	}
-	blk_finish_plug(&plug);
 
 	kiocb_batch_free(&batch);
 	put_ioctx(ctx);