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Date: Tue, 9 Apr 2013 23:57:51 +0400
From: Gleb Smirnoff <glebius@FreeBSD.org>
To: John Baldwin <jhb@freebsd.org>
Subject: Re: svn commit: r249265 - in head/sys: amd64/include arm/include
 i386/include ia64/include mips/include powerpc/include sparc64/include sys
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On Tue, Apr 09, 2013 at 03:20:25PM -0400, John Baldwin wrote:
J> > J> > Log:
J> > J> >   Merge from projects/counters:
J> > J> >   
J> > J> >     Pad struct pcpu so that its size is denominator of PAGE_SIZE. This
J> > J> >   is done to reduce memory waste in UMA_PCPU_ZONE zones.
J> > J> >   
J> > J> >   Sponsored by:	Nginx, Inc.
J> > J> > 
J> > J> > Modified: head/sys/sys/pcpu.h
J> > J> > 
J> > J> ==============================================================================
J> > J> > --- head/sys/sys/pcpu.h	Mon Apr  8 19:10:45 2013	(r249264)
J> > J> > +++ head/sys/sys/pcpu.h	Mon Apr  8 19:19:10 2013	(r249265)
J> > J> > @@ -180,6 +180,14 @@ struct pcpu {
J> > J> >  	PCPU_MD_FIELDS;
J> > J> >  } __aligned(CACHE_LINE_SIZE);
J> > J> 
J> > J> Why not change this to be PAGE_SIZE rather than putting explicit padding in 
J> > J> all the pcpu fields?  That would seem to be more maintainable.
J> > 
J> > Because it can be smaller than PAGE_SIZE. Two times smaller, four times smaller.
J> 
J> Ugh, I didn't parse the "denominator" part correctly.  I had read it as
J> "multiple".  OTOH, it seems quite cheap even on a 32-bit platform to lose
J> 3k per core (assuming 4k page size and 1k pcpu).

There could be several UMA_ZONE_PCPU zones.

-- 
Totus tuus, Glebius.