[dtrace-discuss] How to record waiters on mutex locks ?

[Jonathan Adams]

On 7/18/06, David Jafferian - Sun Microsystems <david.jafferian at east.sun.com>
wrote:
>
>
> Hi -
>
> I wrote the following simple script :
>
> ---begin---
> #!/usr/sbin/dtrace -s
>
> BEGIN
> {
>         lp = (int64_t)&`hat_statlock;
> }
>
> fbt::mutex_vector_enter:entry
> /arg0 == lp/
> {
>         waiters++;
> }
>
> lockstat:::adaptive-block
> /arg0 == lp/
> {
>         waiters--;
> }
>
> lockstat:::adaptive-spin
> /arg0 == lp/
> {
>         waiters--;
> }
>
> tick-10sec
> {
>         trace(waiters);
> }
> ---end---
>
> Then I tested it on a two CPU sun4u machine as follows :

...

> I don't see why "waiters" should increase over time.


Global variables are not guaranteed to be consistant if modified from
multiple threads.  Try using an aggregation:

s/waiters++/@a = sum(1)/
s/waiters--/@a = sum(-1)/
s/trace(waiters)/printa(@a)/

Cheers,
- jonathan
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