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

NeilBrown
2025-01-27T01:23:57Z

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