netdev - RE: RFC: memory leak in udp_table

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Date:	Thu, 1 Mar 2012 08:55:01 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Eric Dumazet" <eric.dumazet@...il.com>,
	"David Miller" <davem@...emloft.net>,
	<paul.gortmaker@...driver.com>, <tim.bird@...sony.com>,
	<kuznet@....inr.ac.ru>, <linux-kernel@...r.kernel.org>,
	<netdev@...r.kernel.org>
Subject: RE: RFC: memory leak in udp_table_init

> > The pid table is a good example of something where a hash
> > table is unnecessary.
> > Linux should steal the code I put into NetBSD :-)
> 
> On this unrelated topic.  What algorithm did you use on NetBSD for
> dealing with pids?

Basically I forced the hash chain length to one by allocating
a pid that hit an empty entry in the table.

So you start off with (say) 64 entries and use the low 6
bits to index the table. The higher bits are incremented
each time a 'slot' is reused.
Free entries are kept in a FIFO list.
So each entry either contains a pointer to the process,
or the high bits and the index of the next free slot.
(and the PGID pointer).
When there are only (say) 2 free entries, then the table
size is doubled, the pointers moved to the correct places,
the free ist fixed up, and the minimum number of free entries
doubled.

The overall effect:
- lookup is only ever a mask and index + compare.
- Allocate is always fast and fixed cost (except when
  the table size has to be doubled).
- A pid value will never be reused within (about) 2000
  allocates (for 16bit pids, much larger for 32bit ones).
- Allocated pid numbers tend to be random, certainly
  very difficult to predict.
- Small memory footprint for small systems.
For pids we normally avoid issuing large values, but
will do so to avoid immediate re-use on systems that
have 1000s of active processes.

See lines 580-820 of
http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/kern/kern_proc.c?annotate=1.
182&only_with_tag=MAIN

	David

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