watchdogd, jemalloc, and mlockall

[Warner Losh]

On Nov 3, 2012, at 12:50 PM, Ian Lepore wrote:

> On Sat, 2012-11-03 at 20:41 +0200, Konstantin Belousov wrote:
>> On Sat, Nov 03, 2012 at 12:38:39PM -0600, Ian Lepore wrote:
>>> In an attempt to un-hijack the thread about memory usage increase
>>> between 6.4 and 9.x, I'm starting a new thread here related to my recent
>>> discovery that watchdogd uses a lot more memory since it began using
>>> mlockall(2).
>>> 
>>> I tried statically linking watchdogd and it made a small difference in
>>> RSS, presumably because it doesn't wire down all of libc and libm.
>>> 
>>> VSZ   RSS
>>> 10236 10164  Dynamic
>>> 8624  8636  Static
>>> 
>>> Those numbers are from ps -u on an arm platform.  I just updated the PR
>>> (bin/173332) with some procstat -v output comparing with/without
>>> mlockall().
>>> 
>>> It appears that the bulk of the new RSS bloat comes from jemalloc
>>> allocating vmspace in 8MB chunks.  With mlockall(MCL_FUTURE) in effect
>>> that leads to wiring 8MB to satisfy what probably amounts to a few
>>> hundred bytes of malloc'd memory.
>>> 
>>> It would probably also be a good idea to remove the floating point from
>>> watchdogd to avoid wiring all of libm.  The floating point is used just
>>> to turn the timeout-in-seconds into a power-of-two-nanoseconds value.
>>> There's probably a reasonably efficient way to do that without calling
>>> log(), considering that it only happens once at program startup.
>> 
>> No, I propose to add a switch to turn on/off the mlockall() call.
>> I have no opinion on the default value of the suggested switch.
> 
> In a patch I submitted along with the PR, I added code to query the
> vm.swap_enabled sysctl and only call mlockall() when swapping is
> enabled.  
> 
> Nobody yet has said anything about what seems to me to be the real
> problem here:  jemalloc grabs 8MB at a time even if you only need to
> malloc a few bytes, and there appears to be no way to control that
> behavior.  Or maybe there's a knob in there that didn't jump out at me
> on a quick glance through the header files.

Isn't that only for non-production builds?

Warner