Date: Wed, 09 Jun 2010 08:34:40 +0100 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: Eitan Adler <lists@eitanadler.com> Cc: questions@freebsd.org Subject: Re: why does ps |grep sometimes not return itself? Message-ID: <4C0F4410.40900@infracaninophile.co.uk> In-Reply-To: <AANLkTimdtot-1vlYofXo-dv3V7jyFW7VU0D6dq3vqBqV@mail.gmail.com> References: <AANLkTimdtot-1vlYofXo-dv3V7jyFW7VU0D6dq3vqBqV@mail.gmail.com>
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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 09/06/2010 08:15:23, Eitan Adler wrote: > Why do I sometimes see the grep in ps's output and sometimes not see it? > [eitan@AlphaBeta ~ ]% ps aux|grep Me > eitan 96325 0.0 0.0 1856 724 5 RL+ 10:14AM 0:00.00 grep Me > [eitan@AlphaBeta ~ ]% ps aux|grep Me > [eitan@AlphaBeta ~ !1! ]% > When you run that pipeline the OS doesn't start both programs exactly simultaneously. It starts ps(1) first, then grep(1) together with creating the pipeline by connecting ps's stdout to grep's stdin. Depending on system load and various other factors, this may allow ps(1) to grab the snapshot of the process table that it works on before grep(1) has started. It's a race condition. Whether you see this effect or not will depend very much on system conformation and load. I can't reproduce the effect on a lightly loaded dual processor machine, which always shows the grep process, whereas on a single processor virtual machine running under VirtualBox, I never see grep in the ps output unless I renice the ps(1) process. Cheers Matthew - -- Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard Flat 3 PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate JID: matthew@infracaninophile.co.uk Kent, CT11 9PW -----BEGIN PGP SIGNATURE----- Version: GnuPG/MacGPG2 v2.0.14 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkwPRBAACgkQ8Mjk52CukIxkWgCeNSQX37XoGvaZn2A/0vYrmUka 1yMAnjtZJoYNqOdo9UDkWks/4cYpVFPy =Rkg3 -----END PGP SIGNATURE-----
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