Re: NFS client latencies

[Ingo Molnar <mingo@elte.hu>]

* Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

> > your patch works fine here - but there are still latencies in 
> > nfs_scan_commit()/nfs_scan_list(): see the attached 3.7 msec latency 
> > trace. It happened during a simple big-file writeout and is easily 
> > reproducible. Could the nfsi->commit list searching be replaced with a 
> > radix based approach too?
> 
> That would be 100% pure overhead. The nfsi->commit list does not need 
> to be sorted and with these patches applied, it no longer is. In fact 
> one of the cleanups I still need to do is to get rid of those 
> redundant checks on wb->index (start is now always set to 0, and end 
> is always ~0UL).
> 
> So the overhead you are currently seeing should just be that of 
> iterating through the list, locking said requests and adding them to 
> our private list.

ah - cool! This was a 100 MB writeout so having 3.7 msecs to process 
20K+ pages is not unreasonable. To break the latency, can i just do a 
simple lock-break, via the patch below?
 
	Ingo

--- linux/fs/nfs/pagelist.c.orig
+++ linux/fs/nfs/pagelist.c
@@ -311,8 +311,9 @@ out:
  * You must be holding the inode's req_lock when calling this function
  */
 int
-nfs_scan_list(struct list_head *head, struct list_head *dst,
-	      unsigned long idx_start, unsigned int npages)
+nfs_scan_list(struct nfs_inode *nfsi, struct list_head *head,
+	      struct list_head *dst, unsigned long idx_start,
+	      unsigned int npages)
 {
 	struct list_head	*pos, *tmp;
 	struct nfs_page		*req;
@@ -327,6 +328,8 @@ nfs_scan_list(struct list_head *head, st
 
 	list_for_each_safe(pos, tmp, head) {
 
+		cond_resched_lock(&nfsi->req_lock);
+
 		req = nfs_list_entry(pos);
 
 		if (req->wb_index < idx_start)
--- linux/fs/nfs/write.c.orig
+++ linux/fs/nfs/write.c
@@ -569,7 +569,7 @@ nfs_scan_commit(struct inode *inode, str
 	int res = 0;
 
 	if (nfsi->ncommit != 0) {
-		res = nfs_scan_list(&nfsi->commit, dst, idx_start, npages);
+		res = nfs_scan_list(nfsi, &nfsi->commit, dst, idx_start, npages);
 		nfsi->ncommit -= res;
 		if ((nfsi->ncommit == 0) != list_empty(&nfsi->commit))
 			printk(KERN_ERR "NFS: desynchronized value of nfs_i.ncommit.\n");
--- linux/include/linux/nfs_page.h.orig
+++ linux/include/linux/nfs_page.h
@@ -63,8 +63,8 @@ extern	void nfs_release_request(struct n
 
 extern  int nfs_scan_lock_dirty(struct nfs_inode *nfsi, struct list_head *dst,
 				unsigned long idx_start, unsigned int npages);
-extern	int nfs_scan_list(struct list_head *, struct list_head *,
-			  unsigned long, unsigned int);
+extern	int nfs_scan_list(struct nfs_inode *nfsi, struct list_head *,
+			  struct list_head *, unsigned long, unsigned int);
 extern	int nfs_coalesce_requests(struct list_head *, struct list_head *,
 				  unsigned int);
 extern  int nfs_wait_on_request(struct nfs_page *);