linux-kernel - Re: [PATCH 6/6] selinux: improve symtab string hashing

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Message-ID: <CAEjxPJ4-zG8Z-KW1VXMHQ4x_AgzsdjpC3W7HfyimSKtsinBmYQ@mail.gmail.com>
Date:   Fri, 8 Sep 2023 14:49:00 -0400
From:   Stephen Smalley <stephen.smalley.work@...il.com>
To:     Christian Göttsche <cgzones@...glemail.com>
Cc:     selinux@...r.kernel.org, Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] selinux: improve symtab string hashing

On Fri, Aug 18, 2023 at 11:12 AM Christian Göttsche
<cgzones@...glemail.com> wrote:
>
> The number of buckets is calculated by performing a binary AND against
> the mask of the hash table, which is one less than its size (which is a
> power of two).  This leads to all top bits being discarded, requiring
> for short or similar inputs a hash function with a good avalanche
> effect.
>
> Use djb2a:
>
>     # current
>     common prefixes:  7 entries and 5/8 buckets used, longest chain length 2, sum of chain length^2 11
>     classes:  134 entries and 100/256 buckets used, longest chain length 5, sum of chain length^2 234
>     roles:  15 entries and 6/16 buckets used, longest chain length 5, sum of chain length^2 57
>     types:  4448 entries and 3016/8192 buckets used, longest chain length 41, sum of chain length^2 14922
>     users:  7 entries and 3/8 buckets used, longest chain length 3, sum of chain length^2 17
>     bools:  306 entries and 221/512 buckets used, longest chain length 4, sum of chain length^2 524
>     levels:  1 entries and 1/1 buckets used, longest chain length 1, sum of chain length^2 1
>     categories:  1024 entries and 400/1024 buckets used, longest chain length 4, sum of chain length^2 2740
>
>     # patch
>     common prefixes:  7 entries and 5/8 buckets used, longest chain length 2, sum of chain length^2 11
>     classes:  134 entries and 101/256 buckets used, longest chain length 3, sum of chain length^2 210
>     roles:  15 entries and 9/16 buckets used, longest chain length 3, sum of chain length^2 31
>     types:  4448 entries and 3459/8192 buckets used, longest chain length 5, sum of chain length^2 6778
>     users:  7 entries and 5/8 buckets used, longest chain length 3, sum of chain length^2 13
>     bools:  306 entries and 236/512 buckets used, longest chain length 5, sum of chain length^2 470
>     levels:  1 entries and 1/1 buckets used, longest chain length 1, sum of chain length^2 1
>     categories:  1024 entries and 518/1024 buckets used, longest chain length 7, sum of chain length^2 2992
>
> Signed-off-by: Christian Göttsche <cgzones@...glemail.com>

Assuming this is in fact public domain,
Reviewed-by: Stephen Smalley <stephen.smalley.work@...il.com>