linux-kernel - Re: [GIT PULL] Please pull NFS client changes for Linux 5.18

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Date:   Wed, 30 Mar 2022 20:01:47 +0000
From:   Trond Myklebust <trondmy@...merspace.com>
To:     "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
CC:     "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] Please pull NFS client changes for Linux 5.18

On Wed, 2022-03-30 at 11:17 -0700, Linus Torvalds wrote:
> On Tue, Mar 29, 2022 at 12:36 PM Trond Myklebust
> <trondmy@...merspace.com> wrote:
> > 
> > - Readdir fixes to improve cacheability of large directories when
> > there
> >   are multiple readers and writers.
> 
> So I only took a look at this part now. I've obviously already pulled
> it, but that use of 'xxhash()' just made me go "Whaaa?"
> 
> It's claimed that it's used because of its extreme performance, but
> the performance numbers come from using it as a block hash.
> 
> That's not what nfs does.
> 
> The nfs code just does
> 
>     xxhash(&cookie, sizeof(cookie), 0) & NFS_READDIR_COOKIE_MASK;
> 
> where that "cookie" is just a 64-bit entity. And then it masks off
> everything but 18 bits.
> 
> Is that *really* appropriate use of a new hash function?
> 
> Why is this not just doing
> 
>   #include <hash.h>
> 
>   hash_64(cookie, 18);
> 
> which is a lot more obvious than xxhash().
> 
> If there really are some serious problems with the perfectly standard
> hash() functionality, I think you should document them.
> 
> Because just randomly picking xxhash() without explaining _why_ you
> can't just use the same simple thing we use elsewhere is very odd.
> 
> Or rather, when the only documentation is "performance", then I think
> the regular "hash_64()" is the obvious and trivial choice.
> 
>                   Linus

My main worry was that hash_64() would have too many collisions. Since
this is using cuckoo nesting, that would be a problem.

I did some quick studies after I got your email, and it seems as if my
concerns were unfounded. I've tested both a linear index and a sample
of ext4 getdents offsets.
While the sample of ext4 offsets did show a larger number of collisions
than a simple linear index, it wasn't too terrible (3 collisions in a
sample of 9000 entries).
The linear index showed no collisions up to about 100000 entries.

So, I'd be OK with changing to hash_64() and will queue up a bugfix for
it. I should have done this study earlier...

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@...merspace.com