cpufreq_curr_sysctl: memory allocation

Alexander Motin mav at FreeBSD.org
Sun Jun 20 16:09:45 UTC 2010

Andriy Gapon wrote:
> I noticed that cpufreq_curr_sysctl performs a substantial memory allocation and
> deallocation on each call.  Its size is CF_MAX_LEVELS * sizeof(*levels), which
> is ~24KB.  This happens even for read-only calls to just query current level.
> And such calls happen quite frequently when powerd is running.

Worse is that it not just consumes time, but causes a bunch or TLB flush
IPIs on free(). For read-only call it doesn't even needs CF_MAX_LEVELS *
sizeof(*levels). sizeof(*levels) seems should be enough there. May be
then it fits into some existing UMA zone, minimizing penalty.

> I think that this is an unnecessary and avoidable load for VM system.
> Couldn't a buffer be preallocated in sc and re-used for the calls?
> Even if not, for some reason, then wouldn't it be better to have a dedicated uma
> zone for that rather than doing malloc+free?

Dedicated rarely used UMA zone may eat much more memory then it is
needed on SMP.

Alexander Motin

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