From: "David Laight" <David.Laight@ACULAB.COM>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Eric Dumazet" <eric.dumazet@gmail.com>,
"David Miller" <davem@davemloft.net>,
<paul.gortmaker@windriver.com>, <tim.bird@am.sony.com>,
<kuznet@ms2.inr.ac.ru>, <linux-kernel@vger.kernel.org>,
<netdev@vger.kernel.org>
Subject: RE: RFC: memory leak in udp_table_init
Date: Thu, 1 Mar 2012 08:55:01 -0000 [thread overview]
Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B6E85@saturn3.aculab.com> (raw)
In-Reply-To: <877gz5ze82.fsf@xmission.com>
> > 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
next prev parent reply other threads:[~2012-03-01 8:55 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-02-25 0:55 RFC: memory leak in udp_table_init Tim Bird
2012-02-25 1:27 ` Paul Gortmaker
2012-02-25 5:19 ` Eric Dumazet
2012-02-26 19:20 ` David Miller
2012-02-27 5:40 ` Eric Dumazet
2012-02-27 5:44 ` David Miller
2012-02-27 11:33 ` David Laight
2012-02-29 18:28 ` Eric W. Biederman
2012-03-01 8:55 ` David Laight [this message]
2012-03-01 12:33 ` Eric Dumazet
2012-03-15 17:44 ` Paul E. McKenney
2012-03-02 0:12 ` Eric W. Biederman
2012-02-27 6:03 ` [PATCH v2] mm: add a low limit to alloc_large_system_hash Eric Dumazet
2012-02-27 6:45 ` David Miller
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=AE90C24D6B3A694183C094C60CF0A2F6026B6E85@saturn3.aculab.com \
--to=david.laight@aculab.com \
--cc=davem@davemloft.net \
--cc=ebiederm@xmission.com \
--cc=eric.dumazet@gmail.com \
--cc=kuznet@ms2.inr.ac.ru \
--cc=linux-kernel@vger.kernel.org \
--cc=netdev@vger.kernel.org \
--cc=paul.gortmaker@windriver.com \
--cc=tim.bird@am.sony.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).