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Date:   Thu, 8 Nov 2018 06:07:07 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     'Martin Steigerwald' <martin@lichtvoll.de>,
        Michal Hocko <mhocko@kernel.org>,
        Daniel Colascione <dancol@google.com>,
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        Tim Murray <timmurray@google.com>,
        Joel Fernandes <joelaf@google.com>,
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        Jonathan Corbet <corbet@lwn.net>,
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        Mike Rapoport <rppt@linux.vnet.ibm.com>,
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        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
        Prashant Dhamdhere <pdhamdhe@redhat.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v2] Document /proc/pid PID reuse behavior
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On Thu, Nov 08, 2018 at 01:42:41PM +0000, David Laight wrote:
> From: Matthew Wilcox
> > On Thu, Nov 08, 2018 at 12:02:49PM +0000, David Laight wrote:
> > > This can be mitigated by only allocating 'big' numbers on systems
> > > that have a lot of pids.
> > > You also really want an O(1) allocator.
> > 
> > The allocator is O(log n) -- it's the IDR allocator, used in cyclic mode.
> > n in this case is the highest ID which is still in use.  The tree is
> > log_64(n) levels high.  It walks to the bottom of the tree and puts a
> > pointer into the tree.  If the cursor has wrapped to the beginning of
> > the tree, it may encounter a PID which is still in use; if it does,
> > it does a bitmap scan of that node, and will then walk up the tree,
> > doing a bitmap scan forward at each level until it finds a free PID.
> 
> Right, but you can choose the pid so that you get a perfect hash.
> You can then put a FIFO free list through the unused entries of
> the hash table (just an array).
> Then pid allocate just picks the oldest free entry and ups the
> high bits (that the hash masks out) to make the old value stale.
> Almost no cache lines are involved in the whole operation.

You'd be looking for something like dynamic perfect hashing then?
I think that'd make a great project for someone to try out.  I don't
have the time to look into it myself, and I'm not convinced there's
a real workload that would benefit.  But it'd be a great project for
a university student.