RFR(M): 8166970: Adapt mutex padding according to DEFAULT_CACHE_LINE_SIZE

[Doerr, Martin]

Hi Coleen,

thank you very much.

Thomas is currently out. The idea to use offsetof wouldn't work because it can only be computed after the layout of the class is computed. (_name[MONITOR_NAME_LEN] is a field of the Monitor class in the original implementation. The MONITOR_NAME_LEN is already needed to compute the class layout.)

Just for information:
The sizes appear to be:
debug build on linux PPC64le:
sizeof(MonitorBase):96, sizeof(Monitor):160, CACHE_LINE_PADDING:32 (no padding used because _name would get less than 64 characters)
product build on linux PPC64le:
sizeof(MonitorBase):56, sizeof(Monitor):128, CACHE_LINE_PADDING:72 (the length of _name gets extended from 64 to 72)

Hence, the change is also relevant for platforms with DEFAULT_CACHE_LINE_SIZE=128 (like PPC64).
A large amount of padding only gets inserted on s390 where we have DEFAULT_CACHE_LINE_SIZE=256.

Maybe someone else wants to review the change. (Thomas is not an official reviewer.)

Thanks and best regards,
Martin


-----Original Message-----
From: Coleen Phillimore [mailto:coleen.phillimore at oracle.com] 
Sent: Montag, 3. Oktober 2016 23:33
To: Doerr, Martin <martin.doerr at sap.com>
Subject: Re: RFR(M): 8166970: Adapt mutex padding according to DEFAULT_CACHE_LINE_SIZE


Hi Martin,

This change was approved for JDK9 so I can sponsor it for you anytime.  
I don't know if Thomas Steufe was a reviewer or not.  I think I prefer 
the way you did it to his suggestion, because I like subclasses.

I think you still need another reviewer though, then commit the change 
and send me the export file (so it has your comments, etc in it).

thanks,
Coleen


On 9/30/16 11:48 AM, Doerr, Martin wrote:
> Hi,
>
> the current implementation of Monitor padding (mutex.cpp) assumes that cache lines are 64 Bytes. There's a platform dependent define "DEFAULT_CACHE_LINE_SIZE" available which can be used. Purpose of padding is to avoid false sharing.
>
> My proposed change is here:
> http://cr.openjdk.java.net/~mdoerr/8166970_mutex_padding/webrev.00/
>
> Please review. If will also need a sponsor.
>
> Thanks and best regards,
> Martin
>