Date: Thu, 05 Jun 2008 22:09:25 -0500 From: Kirk Strauser <kirk@strauser.com> To: Kris Kennaway <kris@FreeBSD.org> Cc: freebsd-questions@freebsd.org Subject: Re: Poor read() performance, and I can't profile it Message-ID: <4848AA65.9060600@strauser.com> In-Reply-To: <48489222.7020501@FreeBSD.org> References: <200806051508.29424.kirk@strauser.com> <4848523E.2010604@FreeBSD.org> <200806051617.54400.kirk@strauser.com> <484867E3.3070705@FreeBSD.org> <4848757B.30408@strauser.com> <48489222.7020501@FreeBSD.org>
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Kris Kennaway wrote: > I don't understand what you meant by "It's also doing a lot of lseek()s > to what is likely the current position anyway (example: seek to 0x00, > read 16 bytes, seek to 0x10, etc.)." then. I just meant that 16 was a smaller number than 4096 to use in an example. :-) But anyway, it looks like I was wrong. Each record in this test file is 144 bytes long, but instead of reading 144 bytes, it's reading 4096 bytes then seeking backward 3952 (4096-144) bytes to the start of the next record. For instance: 99823 dumprecspg CALL lseek(0x3,0x1c8,SEEK_SET,0) 99823 dumprecspg CALL read(0x3,0x8106000,0x1000) 99823 dumprecspg CALL lseek(0x3,0x258,SEEK_SET,0) 99823 dumprecspg CALL read(0x3,0x8106000,0x1000) 99823 dumprecspg CALL lseek(0x3,0x2e8,SEEK_SET,0) 99823 dumprecspg CALL read(0x3,0x8106000,0x1000) 99823 dumprecspg CALL lseek(0x3,0x378,SEEK_SET,0) 99823 dumprecspg CALL read(0x3,0x8106000,0x1000) Now, I know this is suboptimal. My code is a patch on another, longer-established project that I wasn't a part of, and I probably can't do a lot about it without a pretty major rewrite. Still, I can't believe the same code is *so* much faster on Linux. I'd also swear that this is a regression and that it used to run much faster on the same FreeBSD machine back when it was running 6.x, but I never bothered to benchmark it then because it didn't seem to be an issue. -- Kirk Strauser
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