Date: Wed, 22 Jun 2011 15:53:10 +0200 From: Attilio Rao <attilio@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: src-committers@freebsd.org, Alan Cox <alc@freebsd.org>, svn-src-all@freebsd.org, "Bjoern A. Zeeb" <bz@freebsd.org>, Bruce Evans <brde@optusnet.com.au>, svn-src-head@freebsd.org Subject: Re: svn commit: r223307 - head/sys/vm Message-ID: <BANLkTinEg=h59RhrhnbGNRniyQ_Sn8W8SA@mail.gmail.com> In-Reply-To: <201106220948.02421.jhb@freebsd.org> References: <201106191913.p5JJDOqJ006272@svn.freebsd.org> <BBC34F79-FFA7-4A05-83B3-DE17E0AB14D0@FreeBSD.org> <20110622063258.D2275@besplex.bde.org> <201106220948.02421.jhb@freebsd.org>
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2011/6/22 John Baldwin <jhb@freebsd.org>: > On Tuesday, June 21, 2011 4:58:10 pm Bruce Evans wrote: >> On Tue, 21 Jun 2011, Bjoern A. Zeeb wrote: >> >> > On Jun 19, 2011, at 7:13 PM, Alan Cox wrote: >> > >> > Hi Alan, >> > >> >> Author: alc >> >> Date: Sun Jun 19 19:13:24 2011 >> >> New Revision: 223307 >> >> URL: http://svn.freebsd.org/changeset/base/223307 >> >> >> >> Log: >> >> =C2=A0Precisely document the synchronization rules for the page's dir= ty field. >> >> =C2=A0(Saying that the lock on the object that the page belongs to mu= st be held >> >> =C2=A0only represents one aspect of the rules.) >> >> >> >> =C2=A0Eliminate the use of the page queues lock for atomically perfor= ming read- >> >> =C2=A0modify-write operations on the dirty field when the underlying = architecture >> >> =C2=A0supports atomic operations on char and short types. >> >> >> >> =C2=A0Document the fact that 32KB pages aren't really supported. >> > >> > contrary to the tinderbox I'd like to point out that all mips kernels = built by universe are broken with a SVN HEAD from earlier today. =C2=A0Coul= d > you please check and see if you can fix it? =C2=A0The errors I get are: >> > >> > vm_page.o: In function `vm_page_clear_dirty': >> > /sys/vm/vm_page.c:(.text+0x18d0): undefined reference to `atomic_clear= _8' >> > /sys/vm/vm_page.c:(.text+0x18d0): relocation truncated to fit: R_MIPS_= 26 against `atomic_clear_8' >> > vm_page.o: In function `vm_page_set_validclean': >> > /sys/vm/vm_page.c:(.text+0x38f0): undefined reference to `atomic_clear= _8' >> > /sys/vm/vm_page.c:(.text+0x38f0): relocation truncated to fit: R_MIPS_= 26 against `atomic_clear_8' >> >> Atomic types shorter than int cannot be used in MI code, since they migh= t >> not exist. =C2=A0Apparently they don't exist on mips. =C2=A0jake@ fixed = all their >> old uses for sparc4 in ~Y2K. > > I agree. =C2=A0Is there any harm in having the 'dirty' and 'valid' fields= in > vm_page always be at least of size 'int'? > > In the case of amd64, vm_page would change from a size of 120 bytes to 12= 8. > > On i386 I think you'd end up changing the size from 68 to 76. > > (Using an int results in alignment padding after 'busy'.) > > Hmm, that's around 120k of extra vm_page_t space for a machine with 64M o= f > RAM, so around 0.18% of RAM would be used on both platforms (presumably t= he > usage would be similar on other platforms as well). =C2=A0At 24 GB of RAM= , the > extra space is just under 0.20% of RAM (48M). I think that there is a bit of misunderstanding here. The idea here is to use just the atomic 8bits for the architectures that support it and otherwise use the vm_page_queue_lock. Anyway, as not all the architectures have this implementation, rather than replicate a lot of MD code to be used in MI, Alan just went with an explicit stub, where necessary. The real bug I see here is that we thought mips was supporting _8 atomic ops while it doesn't seem to be the case (sorry for being lazy on checking, but I nearly have 0 time). Attilio --=20 Peace can only be achieved by understanding - A. Einstein
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