Date: Tue, 9 Apr 2013 09:32:11 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Gleb Smirnoff <glebius@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, 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 Message-ID: <20130409090906.X1317@besplex.bde.org> In-Reply-To: <20130408194436.GI76816@FreeBSD.org> References: <201304081919.r38JJAvB088644@svn.freebsd.org> <201304081541.27408.jhb@freebsd.org> <20130408194436.GI76816@FreeBSD.org>
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On Mon, 8 Apr 2013, 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. I said in private mail that I don't see a better way to do this. Now I see a way that might work: #define (in the MD pcpu.h) the expected number of times smaller (hopefully not fractional). This won't change very often. Then, if large alignments like PAGE_SIZE work, then smaller ones like PAGE_SIZE / <number of times smaller> will also work. How does explicit aligning to only CACHE_LINE_SIZE work anyway? I think you actually need PAGE_SIZE/N, and get this by magic padding combined with explicit alignment to only CACHE_LINE_SIZE. Bruce Bruce
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