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From: John Baldwin <jhb@freebsd.org>
To: Gleb Smirnoff <glebius@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
Date: Tue, 9 Apr 2013 15:20:25 -0400
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On Monday, April 08, 2013 3:44:36 pm Gleb Smirnoff wrote:
> On Mon, Apr 08, 2013 at 03:41:27PM -0400, John Baldwin wrote:
> J> On Monday, April 08, 2013 3:19:10 pm Gleb Smirnoff wrote:
> J> > Author: glebius
> J> > Date: Mon Apr  8 19:19:10 2013
> J> > New Revision: 249265
> J> > URL: http://svnweb.freebsd.org/changeset/base/249265
> J> > 
> J> > Log:
> J> >   Merge from projects/counters:
> J> >   
> J> >     Pad struct pcpu so that its size is denominator of PAGE_SIZE. This
> J> >   is done to reduce memory waste in UMA_PCPU_ZONE zones.
> J> >   
> J> >   Sponsored by:	Nginx, Inc.
> J> > 
> J> > Modified: head/sys/sys/pcpu.h
> J> > 
> J> ==============================================================================
> J> > --- head/sys/sys/pcpu.h	Mon Apr  8 19:10:45 2013	(r249264)
> J> > +++ head/sys/sys/pcpu.h	Mon Apr  8 19:19:10 2013	(r249265)
> J> > @@ -180,6 +180,14 @@ struct pcpu {
> J> >  	PCPU_MD_FIELDS;
> J> >  } __aligned(CACHE_LINE_SIZE);
> J> 
> J> Why not change this to be PAGE_SIZE rather than putting explicit padding in 
> J> all the pcpu fields?  That would seem to be more maintainable.
> 
> Because it can be smaller than PAGE_SIZE. Two times smaller, four times smaller.

Ugh, I didn't parse the "denominator" part correctly.  I had read it as
"multiple".  OTOH, it seems quite cheap even on a 32-bit platform to lose
3k per core (assuming 4k page size and 1k pcpu).

-- 
John Baldwin