[dtrace-discuss] What am I missing in the dtrace script?

[Brian D. Horn]

Bryan Cantrill wrote:
> On Tue, Jul 10, 2007 at 03:27:46PM -0700, Brian D. Horn wrote:
>   
>> I've asked around, I tried searching on-line, but no go.
>>
>> I would think what I am seeing is impossible or that there must
>> be a bug in dtrace on MP systems (at least on 64-bit amd systems)
>> as this was run on a Sun Fire x4500 (4-way Opteron - aka "Thumper")
>>
>> Here is the script:
>>
>>
>> #! /usr/sbin/dtrace -s
>>
>> BEGIN
>> {
>>        total_queued = 0;
>>        queuing_failed = 0;
>>        queuing_succeeded = 0;
>> }
>>
>> ::vdev_queue_io_to_issue:return
>> {
>>        ++total_queued;
>> }
>>
>> ::vdev_queue_io_to_issue:return
>> / args[1] == 0 /
>> {
>>        ++queuing_failed;
>> }
>>
>> ::vdev_queue_io_to_issue:return
>> / args[1] != 0 /
>> {
>>        ++queuing_succeeded;
>> }
>>
>> END
>> {
>>        printf("\nTotal %d succeeded %d failed %d\n",
>>                total_queued, queuing_succeeded, queuing_failed);
>> }
>>
>> And below is the output:
>>
>> ^C
>> CPU     ID                    FUNCTION:NAME
>>  3      2                             :END
>> Total 970818 succeeded 485824 failed 485584
>>
>> Note that:
>>
>> # bc
>> 485824+485584
>> 971408
>>
>> So the sum of the two cases with qualifiers do not sum
>> to the the total.  I've seen this before and running the script
>> with all but one CPU off line seems to "fix" the problem.
>>     
>
> When you're using global variables, they are modified without any locking
> whatsoever -- so those increments are racing with one another on different
> CPUs.  To achieve what you're trying to achieve, use aggregations, and
> then printa() to process them:
>   

BTW, where is it documented that globals are not lock protected but 
aggregates are?
The closest thing I saw as a in a "dos and don'ts" section that is was 
recommended
to use aggregates when possible, but that was about it.
> ::vdev_queue_io_to_issue:return
> {
> 	@total_queued = count();
> }
>
> ::vdev_queue_io_to_issue:return
> /args[1] == 0/
> {
> 	@queuing_failed = count();
> }
>
> ::vdev_queue_io_to_issue:return
> /args[1] != 0/
> {
> 	@queuing_succeeded = count();
> }
>
> END
> {
> 	printa("\nTotal %@d succeeded %@d failed %@d\n",
> 	    @total_queued, @queuing_succeeded, @queuing_failed);
> }
>
> 	- Bryan
>
> --------------------------------------------------------------------------
> Bryan Cantrill, Solaris Kernel Development.       http://blogs.sun.com/bmc
>