Date: Wed, 3 Aug 2005 10:01:17 -0500 From: Dan Nelson <dnelson@allantgroup.com> To: Maxim.Sobolev@portaone.com Cc: "current@freebsd.org" <current@freebsd.org> Subject: Re: Sub-optimal libc's read-ahead buffering behaviour Message-ID: <20050803150117.GD93405@dan.emsphone.com> In-Reply-To: <42F0CCD5.9090200@portaone.com> References: <42F0CCD5.9090200@portaone.com>
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In the last episode (Aug 03), Maxim Sobolev said:
> I have found the scenario in which our libc behaves utterly
> suboptimally. Consider the following piece of code reads and
> processes every other 512-bytes block in a file (error handling
> intentionally omitted):
>
> What I have discovered in this case is that libc reads 4096 bytes
> from the file for *each* fread(3) call, despite the fact that it can
> only do one actual read(2) for every fourth fread(3) and satisfy the
> rest from the internal buffer (4096 bytes). However, if I replace
> fseek(3) with just another dummy fread(3) everything works as
> expected - libc does only one read for every 8 fread(3) calls (4
> dummy and 4 real).
>
> Is it something which should be fixed or are there some subtle
> reasons for the current behaviour?
I don't think stdio buffers character devices. If you try it again,
opening a regular file (I tried with /boot/kernel/kernel), you'll see
much better behaviour. There are still some unnecessary seeks though:
truss ./a.out
open("/boot/kernel/kernel",O_RDONLY,0666) = 3 (0x3)
fstat(3,{mode=-r-xr-xr-x ,inode=12295,size=4372853,blksize=8192}) = 0 (0x0)
readlink("/etc/malloc.conf","AJ",63) = 2 (0x2)
issetugid() = 0 (0x0)
mmap(0x0,4096,PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_ANON,-1,0x0) = 672555008 (0x28166000)
break(0x804b000) = 0 (0x0)
break(0x804d000) = 0 (0x0)
read(3,"\^?ELF\^A\^A\^A\t\0\0\0\0\0\0\0\0\^B\0"...,8192) = 8192 (0x2000)
read(3,"\0\0\0\0\0\0\0\0\M-u\^E\0\0\M-+\^V\0\0q"...,8192) = 8192 (0x2000)
exit(0x0)
truss ./a.out 1
open("/boot/kernel/kernel",O_RDONLY,0666) = 3 (0x3)
fstat(3,{mode=-r-xr-xr-x ,inode=12295,size=4372853,blksize=8192}) = 0 (0x0)
readlink("/etc/malloc.conf","AJ",63) = 2 (0x2)
issetugid() = 0 (0x0)
mmap(0x0,4096,PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_ANON,-1,0x0) = 672555008 (0x28166000)
break(0x804b000) = 0 (0x0)
break(0x804d000) = 0 (0x0)
read(3,"\^?ELF\^A\^A\^A\t\0\0\0\0\0\0\0\0\^B\0"...,8192) = 8192 (0x2000)
lseek(3,0x0,SEEK_CUR) = 8192 (0x2000)
lseek(3,0x2000,SEEK_SET) = 8192 (0x2000)
read(3,"\0\0\0\0\0\0\0\0\M-u\^E\0\0\M-+\^V\0\0q"...,8192) = 8192 (0x2000)
exit(0x0)
--
Dan Nelson
dnelson@allantgroup.com
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