Date: Fri, 21 Oct 2011 14:08:29 +0200 From: Ivan Voras <ivoras@freebsd.org> To: freebsd-hackers@freebsd.org Subject: Re: Measuring memory footprint in C/C++ code on FreeBSD Message-ID: <j7rnbt$nfa$1@dough.gmane.org> In-Reply-To: <4EA15004.50308@gmail.com> References: <4EA0610B.90206@gmail.com> <20111021084413.GA46039@server.vk2pj.dyndns.org> <4EA1471E.9050501@gmail.com> <j7rhc9$bvb$1@dough.gmane.org> <4EA15004.50308@gmail.com>
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This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig75685E8576A963ABF750171C Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 21/10/2011 12:57, Razmig K wrote: > Le 21.10.2011 12:26, Ivan Voras a =C3=A9crit : >> Well, do you know that SIZE in top is virtual memory size, not residen= t >> size (which is the "RES" column)? You can allocate whatever you want >> from virtual memory, it is not "used" until it's touched. >=20 > Yes, I do. So do you suggest using RES as a better indicator of memory > footprint? Almost certainly yes. Measuring virtual memory is significantly less important for real-world loads. Some of this is very nicely described here: https://www.varnish-cache.org/trac/wiki/ArchitectNotes . > The program in question processes large 3D images via vtk, and I'd like= > to measure its memory usgae with different parameter configurations as > the maximum amount of memory acquired during execution. Since SIZE ofte= n > happens to be larger than RES, and increase more during execution, I > thought of using it as an indicator of memory footprint. No; the difference between SIZE and RES is "slack space" - allocated but untouched virtual memory, which is *NOT PRESENT IN RAM*. You can verify this yourself: make a small C program and allocate twice the physical memory (+swap) you have on the machine (try terabytes on a 64-bit machine), and it will succeed. If you look at this program in top, it should (barring some optimizations) show you that SIZE is huge, but RES is a couple of MB, basically like you didn't allocate anything at all. Now, it is a whole other thing if you try to actually *use* this memory you've allocated. Here's a random link on the topic from Google: http://opsmonkey.blogspot.com/2007/01/linux-memory-overcommit.html . Unfortunately, the phrase "memory overcommit" has been hijacked by the virtualization environment to mean the same thing but relating to the memory in virtual machines. --------------enig75685E8576A963ABF750171C Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.18 (FreeBSD) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk6hYL0ACgkQldnAQVacBch22QCg38zjUUahKydX8V5IFbeFXcDB leMAniWcI5UM+w1YoZFlpqu8fstCp93v =SRBe -----END PGP SIGNATURE----- --------------enig75685E8576A963ABF750171C--
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