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From:   Trond Myklebust <trondmy@gmail.com>
To:     Alkis Georgopoulos <alkisg@gmail.com>,
        Anna Schumaker <Anna.Schumaker@netapp.com>
Cc:     linux-nfs@vger.kernel.org
Subject: [PATCH] NFS: Optimise the default readahead size
Date:   Sun, 22 Sep 2019 15:07:49 -0400
Message-Id: <20190922190749.54156-1-trond.myklebust@hammerspace.com>
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Sender: linux-nfs-owner@vger.kernel.org
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In the years since the max readahead size was fixed in NFS, a number of
things have happened:
- Users can now set the value directly using /sys/class/bdi
- NFS max supported block sizes have increased by several orders of
  magnitude from 64K to 1MB.
- Disk access latencies are orders of magnitude faster due to SSD + NVME.

In particular note that if the server is advertising 1MB as the optimal
read size, as that will set the readahead size to 15MB.
Let's therefore adjust down, and try to default to VM_READAHEAD_PAGES.
However let's inform the VM about our preferred block size so that it
can choose to round up in cases where that makes sense.

Reported-by: Alkis Georgopoulos <alkisg@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
---
 fs/nfs/internal.h | 8 --------
 fs/nfs/super.c    | 9 ++++++++-
 2 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index e64f810223be..447a3c17fa8e 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -16,14 +16,6 @@ extern const struct export_operations nfs_export_ops;
 
 struct nfs_string;
 
-/* Maximum number of readahead requests
- * FIXME: this should really be a sysctl so that users may tune it to suit
- *        their needs. People that do NFS over a slow network, might for
- *        instance want to reduce it to something closer to 1 for improved
- *        interactive response.
- */
-#define NFS_MAX_READAHEAD	(RPC_DEF_SLOT_TABLE - 1)
-
 static inline void nfs_attr_check_mountpoint(struct super_block *parent, struct nfs_fattr *fattr)
 {
 	if (!nfs_fsid_equal(&NFS_SB(parent)->fsid, &fattr->fsid))
diff --git a/fs/nfs/super.c b/fs/nfs/super.c
index 703f595dce90..c96194e28692 100644
--- a/fs/nfs/super.c
+++ b/fs/nfs/super.c
@@ -2627,6 +2627,13 @@ int nfs_clone_sb_security(struct super_block *s, struct dentry *mntroot,
 }
 EXPORT_SYMBOL_GPL(nfs_clone_sb_security);
 
+static void nfs_set_readahead(struct backing_dev_info *bdi,
+			      unsigned long iomax_pages)
+{
+	bdi->ra_pages = VM_READAHEAD_PAGES;
+	bdi->io_pages = iomax_pages;
+}
+
 struct dentry *nfs_fs_mount_common(struct nfs_server *server,
 				   int flags, const char *dev_name,
 				   struct nfs_mount_info *mount_info,
@@ -2669,7 +2676,7 @@ struct dentry *nfs_fs_mount_common(struct nfs_server *server,
 			mntroot = ERR_PTR(error);
 			goto error_splat_super;
 		}
-		s->s_bdi->ra_pages = server->rpages * NFS_MAX_READAHEAD;
+		nfs_set_readahead(s->s_bdi, server->rpages);
 		server->super = s;
 	}
 
-- 
2.21.0