On Thu, 13 Jun 2024, Jan Kara wrote:
> Commit 6df25e58532b ("nfs: remove reliance on bdi congestion")
> introduced NFS-private solution for limiting number of writes
> outstanding against a particular server. Unlike previous bdi congestion
> this algorithm actually works and limits number of outstanding writeback
> pages to nfs_congestion_kb which scales with amount of client's memory
> and is capped at 256 MB. As a result some workloads such as random
> buffered writes over NFS got slower (from ~170 MB/s to ~126 MB/s). The
> fio command to reproduce is:
>
> fio --direct=0 --ioengine=sync --thread --invalidate=1 --group_reporting=1
> --runtime=300 --fallocate=posix --ramp_time=10 --new_group --rw=randwrite
> --size=64256m --numjobs=4 --bs=4k --fsync_on_close=1 --end_fsync=1
>
> This happens because the client sends ~256 MB worth of dirty pages to
> the server and any further background writeback request is ignored until
> the number of writeback pages gets below the threshold of 192 MB. By the
> time this happens and clients decides to trigger another round of
> writeback, the server often has no pages to write and the disk is idle.
>
> To fix this problem and make the client react faster to eased congestion
> of the server by blocking waiting for congestion to resolve instead of
> aborting writeback. This improves the random 4k buffered write
> throughput to 184 MB/s.
>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> fs/nfs/client.c | 1 +
> fs/nfs/write.c | 12 ++++++++----
> include/linux/nfs_fs_sb.h | 1 +
> 3 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/fs/nfs/client.c b/fs/nfs/client.c
> index 3b252dceebf5..8286edd6062d 100644
> --- a/fs/nfs/client.c
> +++ b/fs/nfs/client.c
> @@ -994,6 +994,7 @@ struct nfs_server *nfs_alloc_server(void)
>
> server->change_attr_type = NFS4_CHANGE_TYPE_IS_UNDEFINED;
>
> + init_waitqueue_head(&server->write_congestion_wait);
> atomic_long_set(&server->writeback, 0);
>
> ida_init(&server->openowner_id);
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index c6255d7edd3c..21a5f1e90859 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -423,8 +423,10 @@ static void nfs_folio_end_writeback(struct folio *folio)
>
> folio_end_writeback(folio);
> if (atomic_long_dec_return(&nfss->writeback) <
> - NFS_CONGESTION_OFF_THRESH)
> + NFS_CONGESTION_OFF_THRESH) {
> nfss->write_congested = 0;
> + wake_up_all(&nfss->write_congestion_wait);
> + }
> }
>
> static void nfs_page_end_writeback(struct nfs_page *req)
> @@ -698,12 +700,14 @@ int nfs_writepages(struct address_space *mapping, struct writeback_control *wbc)
> struct nfs_pageio_descriptor pgio;
> struct nfs_io_completion *ioc = NULL;
> unsigned int mntflags = NFS_SERVER(inode)->flags;
> + struct nfs_server *nfss = NFS_SERVER(inode);
> int priority = 0;
> int err;
>
> - if (wbc->sync_mode == WB_SYNC_NONE &&
> - NFS_SERVER(inode)->write_congested)
> - return 0;
> + /* Wait with writeback until write congestion eases */
> + if (wbc->sync_mode == WB_SYNC_NONE && nfss->write_congested)
> + wait_event(nfss->write_congestion_wait,
> + nfss->write_congested == 0);
If there is a network problem or the server reboot, this could wait
indefinitely. Maybe wait_event_idle() ??
Is this only called from the writeback thread that is dedicated to this
fs? If so that should be fine. If it can be reached from a user
processes, then wait_event_killable() might be needed.
Thanks,
NeilBrown
>
> nfs_inc_stats(inode, NFSIOS_VFSWRITEPAGES);
>
> diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
> index 92de074e63b9..38a128796a76 100644
> --- a/include/linux/nfs_fs_sb.h
> +++ b/include/linux/nfs_fs_sb.h
> @@ -140,6 +140,7 @@ struct nfs_server {
> struct rpc_clnt * client_acl; /* ACL RPC client handle */
> struct nlm_host *nlm_host; /* NLM client handle */
> struct nfs_iostats __percpu *io_stats; /* I/O statistics */
> + wait_queue_head_t write_congestion_wait; /* wait until write congestion eases */
> atomic_long_t writeback; /* number of writeback pages */
> unsigned int write_congested;/* flag set when writeback gets too high */
> unsigned int flags; /* various flags */
> --
> 2.35.3
>
>
