Re: Strange performance change 59 -> 61/62

["Martin J. Bligh" <mbligh@aracnet.com>]

>> I'm comparing 59-mjb6 to 61-mjb1 and notice some strange performance
>> differences that I can't explain ... not a big drop, but odd.
>> ...
>> 
>> 1562 .text.lock.file_table
>> 583 dentry_open
>> 551 get_empty_filp
> 
> The first one here is fget().  That's causing problems on ppc64 as well - the
> machine is spending as long in fget as it is in copy_foo_user() in dbench
> runs.
> 
> One possibility is that we're calling fget() more often than previously,
> although that would be rather odd.  Can you add the below patch, and monitor
> /proc/meminfo:nr_fgets?

Thanks for the patch, sorry it took me so long to get the testing done.

59: 4742165
61: 4743166

Pretty damned close ;-)

> If not that, then maybe some funny cacheline aliasing thing?

Mmm... you mean like something sharing the cacheline with the file lock?

M.