[PATCH] make nfsd_drc_max_mem configurable

[Christoph Martin <martin@uni-mainz.de>]

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Dear Andy, dear Bruce,

(sorry for the recent to you, I now cc'd linux-nfs)

I have attached a patch to nfssvc.diff to make the size of the drc nfsd
cache configurable.

In the last month we were stumbling twice over the problem that the
NFS4.1 session cache was to small.

The first time we wanted to setup a NFS Server for our HPC cluster. We
were wondering why we were only able to mount the filesystem on 380 of
our ~700 nodes. It took us a long time to find out that it was the limit
of the NFS4.1 session cache. Since this machine had 12G Ram, the kernel
reserved 12M for the cache, which results in 384 slots a 32k:

echo $(((12582912>>10)/32))
384

We patched the kernel redhat 7 kernel to change NFSD_DRC_SIZE_SHIFT to
from 10 to 7 to fix this problem.

The second time we installed a small Debian VM with 1G ram to act as a
NFS4 referral server for the home and group directories on our campus.
Since the server does only NFS referrals it does not really need more
memory than the 1G. But it could only server about 30 clients with this
limitation of the session cache.

I think it would be a good idea to have the amount of memory
configurable in nfsd. So I wrote this small patch to make drc_size
configurable while loading the kernel nfsd module.

The patch uses the old value computed from NFSD_DRC_SIZE_SHIFT as the
lower limit. If drc_size as a parameter for then nfsd is higher than a
1/1000 of the RAM, this value will be used.

One might consider to make NFSD_DRC_SIZE_SHIFT even higher to use less
memory for situations where it is not needed. I did not implement an
upper limit, but it might be important.

Please consider to include this patch into the nfsd code.

Yours
Christoph Martin

--- linux-source-3.16/fs/nfsd/nfssvc.c	2015-03-30 12:09:09.000000000 +0200
+++ linux-source-3.16.nfsd/fs/nfsd/nfssvc.c	2015-06-17
09:28:37.880443867 +0200
@@ -359,11 +359,19 @@ void nfsd_reset_versions(void)
  * For now this is a #defined shift which could be under admin control
  * in the future.
  */
+
+static ulong drc_size = 0;
+module_param(drc_size, ulong, 0444);
+MODULE_PARM_DESC(drc_size,
+		 "size of NFSv4.1 DRC cache memory (default and minimum:
free_buffer_size >> 10)");
+
 static void set_max_drc(void)
 {
 	#define NFSD_DRC_SIZE_SHIFT	10
-	nfsd_drc_max_mem = (nr_free_buffer_pages()
-					>> NFSD_DRC_SIZE_SHIFT) * PAGE_SIZE;
+	nfsd_drc_max_mem = max(drc_size,
+			       (nr_free_buffer_pages()
+				>> NFSD_DRC_SIZE_SHIFT) * PAGE_SIZE);
+	drc_size = nfsd_drc_max_mem;
 	nfsd_drc_mem_used = 0;
 	spin_lock_init(&nfsd_drc_lock);
 	dprintk("%s nfsd_drc_max_mem %lu \n", __func__, nfsd_drc_max_mem);



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