KS2007: Memory management & application notification of low memory [LWN.net]

KS2007: Memory management & application notification of low memory

Posted Sep 13, 2007 22:30 UTC (Thu) by nix (subscriber, #2304)
In reply to: KS2007: Memory management & application notification of low memory by oak
Parent article: KS2007: Memory management

The glibc mmap threshold *minimum* is 128Kb. In recent versions of glibc
the threshold is dynamically adjusted between 128Kb and 512Kb (on 32-bit
boxes) and 64Mb (on 64-bit). (The mmap threshold starts at 128Kb and rises
whenever the application frees mmap()ed memory, so that transient
allocations tend to use brk() instead.)

(This is new behaviour in glibc 2.5.)

(Log in to post comments)

KS2007: Memory management & application notification of low memory

Posted Sep 16, 2007 19:43 UTC (Sun) by oak (subscriber, #2786) [Link]

> The glibc mmap threshold *minimum* is 128Kb. In recent versions of glibc
the threshold is dynamically adjusted [...] (This is new behaviour in
glibc 2.5.)

Can you still limit that to be smaller with MALLOC_MMAP_THRESHOLD_
environment variable? This would indicate that the limit just increases
and never decreases, regardless of the environment variable:
http://sources.redhat.com/cgi-bin/cvsweb.cgi/libc/malloc/...

However, in some environments and with some applications, the memory
fragmentation is actually worse problem than some performance decrease. If
a device (e.g. an embedded one where you cannot just add more RAM) runs
out of memory because of heap fragmentation, that's going to have much
more drastic effect on performance than e.g. threading scalability
mentioned here:
http://sourceware.org/bugzilla/show_bug.cgi?id=1541

KS2007: Memory management & application notification of low memory

Posted Sep 16, 2007 23:11 UTC (Sun) by nix (subscriber, #2304) [Link]

MALLOC_MMAP_THRESHOLD_ still works: it's precisely equivalent to calling
mallopt (M_MMAP_THRESHOLD, ...); setting any of these mmap parameters
disables dynamic adjustment completely.