Re: [PATCH] AIO: Don't plug the I/O queue in do_io_submit()

[Shaohua Li]

2011/12/14 Dave Kleikamp <dave.kleikamp@xxxxxxxxxx>:
> 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@xxxxxxxxxx>
> Cc: linux-aio@xxxxxxxxx
> Cc: Chris Mason <chris.mason@xxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Jeff Moyer <jmoyer@xxxxxxxxxx>
>
> 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);
can you explain why this can help? Note, in 3.1 kernel we now force flush
plug list if the list is too long, which will remove a lot of latency.
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