Le mardi 31 janvier 2012 à 15:59 +0800, Shaohua Li a écrit :
> Herbert Poetzl reported a performance regression since 2.6.39. The test
> is a simple dd read, but with big block size. The reason is:
>
> T1: ra (A, A+128k), (A+128k, A+256k)
> T2: lock_page for page A, submit the 256k
> T3: hit page A+128K, ra (A+256k, A+384). the range isn't submitted
> because of plug and there isn't any lock_page till we hit page A+256k
> because all pages from A to A+256k is in memory
> T4: hit page A+256k, ra (A+384, A+ 512). Because of plug, the range isn't
> submitted again.
> T5: lock_page A+256k, so (A+256k, A+512k) will be submitted. The task is
> waitting for (A+256k, A+512k) finish.
>
> There is no request to disk in T3 and T4, so readahead pipeline breaks.
>
> We really don't need block plug for generic_file_aio_read() for buffered
> I/O. The readahead already has plug and has fine grained control when I/O
> should be submitted. Deleting plug for buffered I/O fixes the regression.
>
> One side effect is plug makes the request size 256k, the size is 128k
> without it. This is because default ra size is 128k and not a reason we
> need plug here.
>
> Signed-off-by: Shaohua Li <shaohua.li@xxxxxxxxx>
> Tested-by: Herbert Poetzl <herbert@xxxxxxxxxxxx>
> Tested-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Hmm, this is not exactly the patch I tested from Wu Fengguang
I'll test this one before adding my "Tested-by: ..."
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 97f49ed..b662757 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1400,15 +1400,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
> unsigned long seg = 0;
> size_t count;
> loff_t *ppos = &iocb->ki_pos;
> - struct blk_plug plug;
>
> count = 0;
> retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
> if (retval)
> return retval;
>
> - blk_start_plug(&plug);
> -
> /* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
> if (filp->f_flags & O_DIRECT) {
> loff_t size;
> @@ -1424,8 +1421,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
> retval = filemap_write_and_wait_range(mapping, pos,
> pos + iov_length(iov, nr_segs) - 1);
> if (!retval) {
> + struct blk_plug plug;
> +
> + blk_start_plug(&plug);
This part was not on the tested patch yesterday.
> retval = mapping->a_ops->direct_IO(READ, iocb,
> iov, pos, nr_segs);
> + blk_finish_plug(&plug);
> }
> if (retval > 0) {
> *ppos = pos + retval;
> @@ -1481,7 +1482,6 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
> break;
> }
> out:
> - blk_finish_plug(&plug);
> return retval;
> }
> EXPORT_SYMBOL(generic_file_aio_read);
>
>
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>