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

Jeff Layton
2025-01-27T14:40:10Z

On Mon, 2025-01-27 at 12:20 +1100, NeilBrown wrote:
> 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);
>

Thanks for doing this.

Reviewed-by: Jeff Layton <jlayton@kernel.org>