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Message-ID: <eb077defaf957f458b0c70c8f012ea15d866257a.camel@kernel.org>
Subject: Re: [PATCH] nfs: Avoid flushing many pages with NFS_FILE_SYNC
From: Jeff Layton <jlayton@kernel.org>
To: Jan Kara <jack@suse.cz>, Trond Myklebust <trondmy@hammerspace.com>
Cc: anna@kernel.org, linux-nfs@vger.kernel.org
Date: Sun, 26 May 2024 07:36:43 -0400
In-Reply-To: <20240524161419.18448-1-jack@suse.cz>
References: <20240524161419.18448-1-jack@suse.cz>
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On Fri, 2024-05-24 at 18:14 +0200, Jan Kara wrote:
> When we are doing WB_SYNC_ALL writeback, nfs submits write requests with
> NFS_FILE_SYNC flag to the server (which then generally treats it as an
> O_SYNC write). This helps to reduce latency for single requests but when
> submitting more requests, additional fsyncs on the server side hurt
> latency. NFS generally avoids this additional overhead by not setting
> NFS_FILE_SYNC if desc->pg_moreio is set.
>=20
> However this logic doesn't always work. When we do random 4k writes to a =
huge
> file and then call fsync(2), each page writeback is going to be sent with
> NFS_FILE_SYNC because after preparing one page for writeback, we start wr=
iting
> back next, nfs_do_writepage() will call nfs_pageio_cond_complete() which =
finds
> the page is not contiguous with previously prepared IO and submits is *wi=
thout*
> setting desc->pg_moreio.  Hence NFS_FILE_SYNC is used resulting in poor
> performance.
>=20
> Fix the problem by setting desc->pg_moreio in nfs_pageio_cond_complete() =
before
> submitting outstanding IO. This improves throughput of
> fsync-after-random-writes on my test SSD from ~70MB/s to ~250MB/s.
>=20
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/nfs/pagelist.c | 5 +++++
>  1 file changed, 5 insertions(+)
>=20
> diff --git a/fs/nfs/pagelist.c b/fs/nfs/pagelist.c
> index 6efb5068c116..040b6b79c75e 100644
> --- a/fs/nfs/pagelist.c
> +++ b/fs/nfs/pagelist.c
> @@ -1545,6 +1545,11 @@ void nfs_pageio_cond_complete(struct nfs_pageio_de=
scriptor *desc, pgoff_t index)
>  					continue;
>  			} else if (index =3D=3D prev->wb_index + 1)
>  				continue;
> +			/*
> +			 * We will submit more requests after these. Indicate
> +			 * this to the underlying layers.
> +			 */
> +			desc->pg_moreio =3D 1;
>  			nfs_pageio_complete(desc);
>  			break;
>  		}

Nice work!

Reviewed-by: Jeff Layton <jlayton@kernel.org>