Cached file read performance with 6.2-PRERELEASE

Mark Kirkwood markir at
Tue Dec 19 20:49:09 PST 2006

> What does the memory-related stats from "top" show you? Did you have any 
> other memory intensive applications running at the time? A random 
> example from one of my systems (1GB RAM):

Thanks, good point - but no - absolutely nothing (machine is freshly 
booted, and the only thing running is this test).

Mem: 4672K Active, 4260K Inact, 20M Wired, 14M Buf, 1974M Free

Mem: 5124K Active, 681M Inact, 126M Wired, 112M Buf, 1191M Free

As I understand it, the pages for the file are cached in Inactive and 
the 112M Buf is essentially a 'window' to access 'em (probably said that 
a little wrong... someone who knows better can hopefully correct me)

> That should give you an idea as to how much RAM is being used for the 
> buffer/block IO cache ("111M Buf" in the above example, as I understand 
> it), and the VM disk cache ("36M Cache" in the above example).
> You might also want to look at:
>     sysctl vfs.
> and see whether or not there is anything there that may affect it. For 
> instance, whether there is a maximum size in terms of files that will be 
> cached...? Someone with more VFS/etc knowledge than I may be able to 
> better advise you there...

Thanks - I'll look into these.

> It might be worthwhile trying with a series of different file size to 
> determine if there is a point where the caching performance drops... I 
> just did a few quick tests on a relatively old machine (2x P3-933Mhz, 
> 1GB RAM)... in this case, /tmp is on a 3ware SATA RAID controller 
> (8xxx?) running RAID1 on two 160gb SATA disks)...

Well that proved to be interesting: anything much bigger than 100M is 
pretty flat at 350MB/s...

Cached file size      read rate (8k blocks)
----------------      ---------
100MB                 510MB/s
150MB                 350MB/s
200MB                 350MB/s
800MB                 350MB/s
1.6Gb                 350MB/s

(Off-topic: a 2.5GB file still gets close to 350MB/s even tho it is - 
obviously - partially cached, helped no doubt by a fast IO system - 
3ware 75xx + 4 disk RAID0, which can do 195MB/s for the uncached 
sections of the file...).



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