[PATCH 5/7] nfsd: filecache: document the arbitrary limit on file-disposes-per-loop

[NeilBrown <neilb@suse.de>]

Rather than having the bare number "8" use a named constant and explain
the tradeoffs that lead to the choice.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/nfsd/filecache.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index 1e90da507152..7264faa57280 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -493,6 +493,21 @@ nfsd_file_dispose_list_delayed(struct list_head *dispose)
 	}
 }
 
+/*
+ * Disposing of files can involve non-trivial work, and they
+ * can appear in batches.  So we don't want to try handling them
+ * all in one thread - if there are lots it would be better to allow
+ * several nfsd threads to handle them in parallel.
+ * On average one RPC request can create at most 1 file to be disposed
+ * so handling one each time around the nfsd loop should keep the list
+ * under control.  However there are often benefits of batching so
+ * 2 at a time will likely be more efficient than 1.  4 more so.
+ * We need to choose a number which will often handle all the files,
+ * but will allow other threads to help when the list gets long.
+ * The current choice is:
+ */
+#define NFSD_FILE_DISPOSE_BATCH	8
+
 /**
  * nfsd_file_net_dispose - deal with nfsd_files waiting to be disposed.
  * @nn: nfsd_net in which to find files to be disposed.
@@ -511,7 +526,8 @@ void nfsd_file_net_dispose(struct nfsd_net *nn)
 		int i;
 
 		spin_lock(&l->lock);
-		for (i = 0; i < 8 && !list_empty(&l->freeme); i++) {
+		for (i = 0; i < NFSD_FILE_DISPOSE_BATCH &&
+		     !list_empty(&l->freeme); i++) {
 			struct nfsd_file *nf = list_first_entry(
 				&l->freeme, struct nfsd_file, nf_lru);
 
-- 
2.47.1