Date: Wed, 25 Jun 2003 21:38:30 -0700 From: Marcel Moolenaar <marcel@xcllnt.net> To: "D. J. Bernstein" <djb@cr.yp.to> Cc: freebsd-performance@freebsd.org Subject: Re: ten thousand small processes Message-ID: <20030626043830.GA33650@dhcp01.pn.xcllnt.net> In-Reply-To: <20030626025029.71392.qmail@cr.yp.to> References: <20030625060629.51087.qmail@cr.yp.to> <20030625023621.N17881-100000@mail.chesapeake.net> <20030625094301.56349.qmail@cr.yp.to> <20030626025029.71392.qmail@cr.yp.to>
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On Thu, Jun 26, 2003 at 02:50:29AM -0000, D. J. Bernstein wrote: > Jon Mini writes: > > I'm sorry, but you are way off here. First of all, caches are *much > > larger* than the size of the processes you are talking about. > > I'm sorry, but you are being misled by a naive model of CPU performance. > On a typical Pentium in our department, the following program becomes > three times faster when SPACING is changed from 4096 to 128: *snip* > >From an asm programmer's perspective, when FreeBSD decides to spread a > small program's variables between > > * the beginning of a data page, > * the beginning of a bss page, > * the beginning of a malloc mmap page, > * the beginning of a heap page, > * the beginning of the next heap page, > * the beginning of yet another heap page, > > et cetera, it is actively trying (with varying degrees of success) to > damage cache performance in exactly the same way that this program does. Just curious: do you happen to know if the performance hit is caused by the second order effect of having the spacing be a multiple of the cache associativity, thereby resulting in thrashing of a few cache lines, and that compacting the code results in a more uniform cache placement? In other words: is it (sec) the spacing that counts or the interaction of a particular "distance" with cache placement? -- Marcel Moolenaar USPA: A-39004 marcel@xcllnt.net
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