Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   "Benjamin Coddington" <bcodding@redhat.com>
To:     trondmy@kernel.org
Cc:     linux-nfs@vger.kernel.org
Subject: Re: [PATCH v9 14/27] NFS: Improve heuristic for readdirplus
Date:   Wed, 09 Mar 2022 12:39:51 -0500
Message-ID: <A7BBBBF2-768E-487C-A890-7E5AF1D40027@redhat.com>
In-Reply-To: <20220227231227.9038-15-trondmy@kernel.org>
References: <20220227231227.9038-1-trondmy@kernel.org>
 <20220227231227.9038-2-trondmy@kernel.org>
 <20220227231227.9038-3-trondmy@kernel.org>
 <20220227231227.9038-4-trondmy@kernel.org>
 <20220227231227.9038-5-trondmy@kernel.org>
 <20220227231227.9038-6-trondmy@kernel.org>
 <20220227231227.9038-7-trondmy@kernel.org>
 <20220227231227.9038-8-trondmy@kernel.org>
 <20220227231227.9038-9-trondmy@kernel.org>
 <20220227231227.9038-10-trondmy@kernel.org>
 <20220227231227.9038-11-trondmy@kernel.org>
 <20220227231227.9038-12-trondmy@kernel.org>
 <20220227231227.9038-13-trondmy@kernel.org>
 <20220227231227.9038-14-trondmy@kernel.org>
 <20220227231227.9038-15-trondmy@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On 27 Feb 2022, at 18:12, trondmy@kernel.org wrote:

> From: Trond Myklebust <trond.myklebust@hammerspace.com>
>
> The heuristic for readdirplus is designed to try to detect 'ls -l' and
> similar patterns. It does so by looking for cache hit/miss patterns in
> both the attribute cache and in the dcache of the files in a given
> directory, and then sets a flag for the readdirplus code to interpret.
>
> The problem with this approach is that a single attribute or dcache miss
> can cause the NFS code to force a refresh of the attributes for the
> entire set of files contained in the directory.
>
> To be able to make a more nuanced decision, let's sample the number of
> hits and misses in the set of open directory descriptors. That allows us
> to set thresholds at which we start preferring READDIRPLUS over regular
> READDIR, or at which we start to force a re-read of the remaining
> readdir cache using READDIRPLUS.

I like this patch very much.

The heuristic doesn't kick-in until "ls -l" makes its second call into
nfs_readdir(), and for my filenames with 8 chars, that means that there are
about 5800 GETATTRs generated before we clean the cache to do more
READDIRPLUS.  That's a large number to compound on connection latency.

We've already got some complaints that folk's 2nd "ls -l" takes "so much
longer" after 1a34c8c9a49e.

Can we possibly limit our first pass through nfs_readdir() so that the
heuristic takes effect sooner?

Ben