RFR (S): 8073204: Determining the desired PLAB size adjusts to the the number of threads at the wrong place

Sangheon Kim sangheon.kim at oracle.com
Mon Apr 6 23:16:17 UTC 2015


Hi Ramki,

Thank you for reviewing this.

On 04/06/2015 03:40 PM, Srinivas Ramakrishna wrote:
> Hi Sangheon --
>
> Does the number of GC workers ever change dynamically within a 
> stop-world GC phase?
Yes.

> If not, you might save a little work by just doing the division once 
> when you decide the number of
> workers at the start of the copying phase, and then use that value for 
> the remainder of the copying
> phase of the collection when you know the number of workers won't 
> change. However, that is just a
> nit, I am sure the work involved in calculating the value at each PLAB 
> refill shouldn't typically be a
> big deal either way.
Right. I was also considered about using the calculated size for same 
workers, but as it's not a big deal I ended up to calculate every time.

>
> I might rename the _desired_plab_sz field to clarify that it's the net 
> value, say by calling it
> _desired_net_plab_sz. (Alternatively, you could change the name of the 
> method
> desired_plab_sz(int) to desired_plab_size_for_n_workers(int), but the 
> former seems better,
> since the parameter already signals the intention for the latter.)
I will change the field name to '_desired_net_plat_sz'.

Thanks,
Sangheon


>
> looks good to me otherwise.
> -- ramki (openjdk: ysr)
>
>
> On Mon, Apr 6, 2015 at 2:40 PM, Sangheon Kim <sangheon.kim at oracle.com 
> <mailto:sangheon.kim at oracle.com>> wrote:
>
>     Hi all,
>
>     Please review this change to determine the desired PLAB size for
>     current gc worker threads.
>
>     Currently we calculate an optimal PLAB size with current number of
>     gc workers.
>     When the number of workers changes dynamically
>     (-XX:+UseDynamicNumberOfGCThreads), the desired PLAB size
>     returned(by desired_plab_sz()) is still tuned to the number of gc
>     workers that has been used previously.
>
>     This change is first calculate the desired PLAB value for a single
>     gc worker and then return desired PLAB size according to the
>     current number of threads.
>
>     CR:
>     https://bugs.openjdk.java.net/browse/JDK-8073204
>
>     Webrev:
>     http://cr.openjdk.java.net/~sangheki/8073204/webrev.01
>     <http://cr.openjdk.java.net/%7Esangheki/8073204/webrev.01>
>
>     Test:
>     JPRT
>
>     Thanks,
>     Sangheon
>
>

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