Sub-optimal libc's read-ahead buffering behaviour

Dan Nelson dnelson at allantgroup.com
Wed Aug 3 15:01:18 GMT 2005


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 at allantgroup.com


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