From: Nicholas Murphy <nmurphy@eecs.harvard.edu>
To: Josh Stone <jistone@redhat.com>
Cc: systemtap@sourceware.org
Subject: Re: associative array synchronization question
Date: Fri, 20 Jul 2012 20:37:00 -0000 [thread overview]
Message-ID: <5096CB06-32BC-4DFD-8B7B-2912830C3573@eecs.harvard.edu> (raw)
In-Reply-To: <5009BB84.5060308@redhat.com>
Thank you! That's clearer.
What does it mean that you "sometimes" fail to take a lock? Do you just skip executing the probe if you determine that you can't get the lock?
Thanks,
Nick
On Jul 20, 2012, at 4:11 PM, Josh Stone wrote:
> On 07/20/2012 12:47 PM, Nicholas Murphy wrote:
>> Sorry for the very basic question:
>
> No need to apologize.
>
>> it says in the documentation that any probe using an associative
>> array (which is necessarily global) will automatically hold a lock on
>> that array for the duration of the probe...is that true regardless of
>> whether the probe actually touches the array? Or does it only lock
>> on first access?
>
> When we take locks, we use the trylock functions (with a little spinning
> and waiting), so we won't ever block forever. That means sometimes we
> can fail to take a lock.
>
> We also chose to make probe handlers atomic, such that if a probe
> handler runs at all, it must run in full. We don't want to get into a
> situation where a lock in the middle of a probe can't be obtained, thus
> ruining that probe's execution flow.
>
> Therefore, all locks needed for a handler are attempted before anything
> is started in that handler, and held until the handler is done. If any
> of the locks can't be obtained, the probe is skipped entirely.
>
> So for your case, where you may be conditionally writing an array, we
> will be grabbing the write lock regardless of the condition (which we
> don't know at that time).
>
> If your data values can fit stats types (various numeric accumulation),
> and you'll be writing values more often than you read, then a stats
> array may be a better locking choice, because it only needs a fully
> exclusive lock when reading instead.
>
> Hope that helps, and feel free to ask followup questions.
>
> Josh
next prev parent reply other threads:[~2012-07-20 20:37 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-07-20 19:47 Nicholas Murphy
2012-07-20 20:12 ` Josh Stone
2012-07-20 20:37 ` Nicholas Murphy [this message]
2012-07-20 21:20 ` Josh Stone
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