Hi,
while tracking down a very interesting interaction between Sun/SAM-FS and Linux NFS clients, we found out that the value of NFS_MAX_READ_AHEAD is to agressive/big for the specific use-case.
For testing, instead of always recompiling the kernel with different values, I came up with the following patch. It introduces a tunable "/proc/sys/fs/nfs/nfs_ra_factor" with possible values between 0-15.
Not sure whether it is actually a good thing to have. Better would be to set the read-ahead factor per filesystem via a mount option.
The patch is against 2.6.24. It applies with offsets against 2.6.25-rc9. In case my mail client messes up the whitespace, the patch is also attached.
Signed-off-by: Martin Knoblauch <[email protected]>
diff -ru linux-2.6.24-orig/fs/nfs/client.c linux-2.6.24-nfs_ra/fs/nfs/client.c
--- linux-2.6.24-orig/fs/nfs/client.c 2008-01-24 23:58:37.000000000 +0100
+++ linux-2.6.24-nfs_ra/fs/nfs/client.c 2008-04-14 16:44:22.000000000 +0200
@@ -601,6 +601,11 @@
}
/*
+ * NFS Client Read-Ahead factor
+*/
+unsigned int nfs_ra_factor;
+
+/*
* Load up the server record from information gained in an fsinfo record
*/
static void nfs_server_set_fsinfo(struct nfs_server *server, struct nfs_fsinfo *fsinfo)
@@ -625,7 +630,9 @@
server->rsize = NFS_MAX_FILE_IO_SIZE;
server->rpages = (server->rsize + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
- server->backing_dev_info.ra_pages = server->rpages * NFS_MAX_READAHEAD;
+ printk(KERN_INFO "nfs_server_set_fsinfo: rsize, wsize, rpages, nfs_ra_factor, ra_pages: %d %d %d %d %d\n",
+ server->rsize,server->wsize,server->rpages,nfs_ra_factor,server->rpages * nfs_ra_factor);
+ server->backing_dev_info.ra_pages = server->rpages * nfs_ra_factor;
if (server->wsize > max_rpc_payload)
server->wsize = max_rpc_payload;
@@ -1359,6 +1366,8 @@
{
struct proc_dir_entry *p;
+ nfs_ra_factor = NFS_MAX_READAHEAD;
+
proc_fs_nfs = proc_mkdir("nfsfs", proc_root_fs);
if (!proc_fs_nfs)
goto error_0;
diff -ru linux-2.6.24-orig/fs/nfs/sysctl.c linux-2.6.24-nfs_ra/fs/nfs/sysctl.c
--- linux-2.6.24-orig/fs/nfs/sysctl.c 2008-01-24 23:58:37.000000000 +0100
+++ linux-2.6.24-nfs_ra/fs/nfs/sysctl.c 2008-04-14 16:15:03.000000000 +0200
@@ -14,9 +14,12 @@
#include <linux/nfs_fs.h>
#include "callback.h"
+#include "internal.h"
static const int nfs_set_port_min = 0;
static const int nfs_set_port_max = 65535;
+static const unsigned int min_nfs_ra_factor = 0;
+static const unsigned int max_nfs_ra_factor = NFS_MAX_READAHEAD;
static struct ctl_table_header *nfs_callback_sysctl_table;
static ctl_table nfs_cb_sysctls[] = {
@@ -58,6 +61,16 @@
.mode = 0644,
.proc_handler = &proc_dointvec,
},
+ {
+ .ctl_name = CTL_UNNUMBERED,
+ .procname = "nfs_ra_factor",
+ .data = &nfs_ra_factor,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec_minmax,
+ .extra1 = (unsigned int *)&min_nfs_ra_factor,
+ .extra2 = (unsigned int *)&max_nfs_ra_factor,
+ },
{ .ctl_name = 0 }
};
diff -ru linux-2.6.24-orig/include/linux/nfs_fs.h linux-2.6.24-nfs_ra/include/linux/nfs_fs.h
--- linux-2.6.24-orig/include/linux/nfs_fs.h 2008-01-24 23:58:37.000000000 +0100
+++ linux-2.6.24-nfs_ra/include/linux/nfs_fs.h 2008-04-14 15:58:57.000000000 +0200
@@ -415,6 +415,11 @@
extern void nfs_writedata_release(void *);
/*
+ * linux/fs/nfs/client.c
+*/
+extern unsigned int nfs_ra_factor;
+
+/*
* Try to write back everything synchronously (but check the
* return value!)
*/
Cheers
Martin
------------------------------------------------------
Martin Knoblauch
email: k n o b i AT knobisoft DOT de
www: http://www.knobisoft.de
On Tue, 2008-04-15 at 06:39 -0700, Martin Knoblauch wrote:
> Hi,
>
> while tracking down a very interesting interaction between Sun/SAM-FS and Linux NFS clients, we found out that the value of NFS_MAX_READ_AHEAD is to agressive/big for the specific use-case.
>
> For testing, instead of always recompiling the kernel with different values, I came up with the following patch. It introduces a tunable "/proc/sys/fs/nfs/nfs_ra_factor" with possible values between 0-15.
>
> Not sure whether it is actually a good thing to have. Better would be to set the read-ahead factor per filesystem via a mount option.
>
> The patch is against 2.6.24. It applies with offsets against 2.6.25-rc9. In case my mail client messes up the whitespace, the patch is also attached.
NFS_MAX_READ_AHEAD just sets an upper limit on the standard vfs/mm-level
page readahead algorithm. If you need finer control over that readahead
algorithm, then the kernel already has full support for the
posix_fadvise() system call.
Cheers
Trond
----- Original Message ----
> From: Trond Myklebust <[email protected]>
> To: Martin Knoblauch <[email protected]>
> Cc: linux-nfs list <[email protected]>
> Sent: Tuesday, April 15, 2008 6:12:32 PM
> Subject: Re: RFC/Patch: Make NFS Readahead tunable
>
>
> On Tue, 2008-04-15 at 06:39 -0700, Martin Knoblauch wrote:
> > Hi,
> >
> > while tracking down a very interesting interaction between Sun/SAM-FS and
> Linux NFS clients, we found out that the value of NFS_MAX_READ_AHEAD is to
> agressive/big for the specific use-case.
> >
> > For testing, instead of always recompiling the kernel with different values,
> I came up with the following patch. It introduces a tunable
> "/proc/sys/fs/nfs/nfs_ra_factor" with possible values between 0-15.
> >
> > Not sure whether it is actually a good thing to have. Better would be to set
> the read-ahead factor per filesystem via a mount option.
> >
> > The patch is against 2.6.24. It applies with offsets against 2.6.25-rc9. In
> case my mail client messes up the whitespace, the patch is also attached.
>
> NFS_MAX_READ_AHEAD just sets an upper limit on the standard vfs/mm-level
> page readahead algorithm. If you need finer control over that readahead
> algorithm, then the kernel already has full support for the
> posix_fadvise() system call.
>
But to use posix_fadvise I would need control over the application. Correct? This is not given in the case I am actually dealing with. The NFS-server is Solaris-10/Sparc/SAM-FS. It turns out that *any* access to an "offline" file from a Linux-NFS client messes (and I mean messes) up the performance of the stager process if the read-ahead window is larger that 2 or three time rsize. Now - I have a pretty clear opinion of "who is at fault", but doing a band-aid on the Linux side may just be the easier thing, at least short/mid-term. Not sure how important the end-customer really is to the makers of SAM-FS.
In general, of course controlling the readahead behaviour from the application is the better solution. anyway, the patch is simple enough to carry around.
Martin
> Cheers
> Trond
>
>
>