jemalloc enhancement for small-memory systems

[Jason Evans]

On Dec 22, 2012, at 11:28 AM, Ian Lepore <freebsd at damnhippie.dyndns.org> wrote:
> When a daemon such as watchdogd uses mlockall(2) on a small-memory
> embedded system, it can end up wiring much of the available ram because
> jemalloc allocates large chunks of vmspace by default.  More background
> info on this can be found in this thread:
> 
> http://lists.freebsd.org/pipermail/freebsd-embedded/2012-November/001679.html
> 
> It's hard to tune jemalloc's allocation behavior for this in a
> machine-independent way because the minimum chunk size depends on
> PAGE_SIZE and other factors internal to jemalloc.  I've created a patch
> that addresses this by defining that lg_chunk:0 is implicitly a request
> to set the chunk size to the smallest value allowable for the machine
> it's running on.  The patch is attached to this PR...
> 
> http://www.freebsd.org/cgi/query-pr.cgi?pr=174641
> 
> Jason, could you please review this and consider incorporating it into
> jemalloc?  Or let us know if there's a better way to handle this
> situation.

Your approach looks good to me.  I just checked in a slightly simpler patch to the upstream jemalloc repository:

	http://www.canonware.com/cgi-bin/gitweb.cgi?p=jemalloc.git;a=commitdiff;h=1bf2743e08ba66cc141e296812839947223e4370

The only real difference is that no warnings are generated for lg_chunk values between 0 and the minimum supported value.

I don't have any short-term plans to update jemalloc in FreeBSD, so feel free to either commit your patch as is, or update it to merge from the upstream patch (which will make the next update a  bit easier for me).

Thanks,
Jason