netdev - Re: [PATCH RFC 29/30] NFSD: Convert the filecache to use rhashtable

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Message-ID: <20220623223320.GG1098723@dread.disaster.area>
Date:   Fri, 24 Jun 2022 08:33:20 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Chuck Lever III <chuck.lever@...cle.com>
Cc:     Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, "tgraf@...g.ch" <tgraf@...g.ch>,
        Jeff Layton <jlayton@...hat.com>
Subject: Re: [PATCH RFC 29/30] NFSD: Convert the filecache to use rhashtable

On Thu, Jun 23, 2022 at 05:27:20PM +0000, Chuck Lever III wrote:
> Also I just found Neil's nice rhashtable explainer:
> 
>    https://lwn.net/Articles/751374/
> 
> Where he writes that:
> 
> > Sometimes you might want a hash table to potentially contain
> > multiple objects for any given key. In that case you can use
> > "rhltables" — rhashtables with lists of objects.
> 
> I believe that is the case for the filecache. The hash value is
> computed based on the inode pointer, and therefore there can be more
> than one nfsd_file object for a particular inode (depending on who
> is opening and for what access). So I think filecache needs to use
> rhltable, not rhashtable. Any thoughts from rhashtable experts?

Huh, I assumed the file cache was just hashing the whole key so that
every object in the rht has it's own unique key and hash and there's
no need to handle multiple objects per key...

What are you trying to optimise by hashing only the inode *pointer*
in the nfsd_file object keyspace?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com