RFR: 8079315: UseCondCardMark broken in conjunction with CMS precleaning

[Andrew Haley]

Hi,

On 05/05/2015 04:02 PM, Vitaly Davidovich wrote:

> I realize this is a correctness fix, but isn't this going to possibly
> defeat any perf gain from using conditional card marking in the first place
> (for CMS)?

It shouldn't do.  The idea of the UseCondCardMark AIUI is to reduce
thrashing of card table cache lines across cores.  StoreLoad, while
possibly slow, should still be a local operation.  So it all depends
on what we're trying to optimize: it's still the right thing to use
on a many-core machine, perhaps not for something smaller.

> Didn't someone suggest a different approach that allows
> store-store to be used still?

Mikael wrote:

> For UseCondCardMark to be correct with CMS with precleaning it would 
> require a StoreLoad between the field write and the card table load.
> 
> Another approach to solve this partially would be to change the 
> condition for the conditional card mark from an equality test aginst 0x0 
> (dirty_card_val)
> to a negated equality test of 0xff (clean_card_val)
> 
> In that case we would slightly reduce the number of false-sharing 
> inducing card writes and still survive the precleaning phase since 
> precleaning sets the card to 0x1 (precleaned_card_val).

But I'm at a loss to understand how that helps with the problem of
reading an old value for the card mark.  There still has to be some
sort of happens-before relationship between something and the read of
the card mark.  What good would a store-store do here?

Andrew.