On Fri, 2 Jun 2023 17:55:56 GMT, Y. Srinivas Ramakrishna <y...@openjdk.org>
wrote:
>> Kelvin Nilsen has updated the pull request incrementally with one additional
>> commit since the last revision:
>>
>> Force PLAB sizes to align on card-table size
>
> src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 1285:
>
>> 1283: if (unalignment != 0) {
>> 1284: word_size = word_size - unalignment +
>> CardTable::card_size_in_words();
>> 1285: }
>
> Probably not a big deal since this is only used when refilling a PLAB, which
> is an infrequent operation, but `mod` is an expensive operation, in general,
> and best to avoid in our code except in assertion checks (or even there given
> recent experiences with debug tests timing out). Since card size is a power
> of 2, may be we could use addition and masking instead. Something like
> defining the following inline in the CardTable class and using it everywhere
> where card alignment granularity is sought. There may even be a macro or
> method defined for this already perhaps:
>
>
> (FOO + CardSize - 1) & ~((1 << LogCardSize) - 1)
>
>
> One could even store the mask to avoid the arithmetic to produce the mask
> although it's pretty cheap.
>
> This may turn out to be less expensive than mod, test, and branch, but as I
> said probably not a big deal here. We should make sure we don't overuse mods
> in our allocation paths much.
Thanks for this suggestion. I've modified the code.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/14185#discussion_r1220523992
