Re: Linux VFS cache hit rate script

[Josh Stone <jistone@redhat.com>]

On 04/20/2011 01:45 PM, Jake Maul wrote:
> On Wed, Apr 20, 2011 at 1:08 PM, William Cohen <wcohen@redhat.com> wrote:
>> Why the need for -DMAXMAPENTRIES=100000 on the second example on
>> the wiki page? There doesn't seem to be any arrays in the script.
> 
> For some reason on that particular server, I get this error sometimes:
> 
> ERROR: Array overflow, check MAXMAPENTRIES near identifier '$file' at
> /usr/share/systemtap/tapset/vfs.stp:769:9

The error location looks bogus, which we should investigate, but this is
probably from the array used in vfs.stp:__find_bdevname().  I'm not sure
how you could be encountering so many unique "dev" though.

>> The following line sounds unsure, need to check that that is really the case:
>>
>>  if (devname=="N/A") { /* N/A means cache hit... right? */
>>
>> It would be good to have a better explanation for the "N/A" check.
> 
> This is from the beginner's guide, actually:
> 
> http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/SystemTap_Beginners_Guide/mainsect-disk.html
> 
> It bothers me too, because I can't find any documentation that says
> explicitly when you would get an 'N/A' devname. I don't know if it's a
> SystemTap thing or a kernel thing, but being that I don't have
> experience working on either, I'm fairly lost.

That string comes from dev.stp:bdevname(bdev), whenever the parameter is
zero.  I'm not sure about the particulars though, whether bdev==0 really
means it's a cache hit...

Josh