Query regarding ordering in G1 post-write barrier (original) (raw)

Andrew Dinn adinn at redhat.com
Mon Jul 20 13:22:28 UTC 2015

$SUBJECT is in relation to the following code in method GraphKit::g1_mark_card()

. . . __ storeCM(__ ctrl(), card_adr, zero, oop_store, oop_alias_idx, card_bt, Compile::AliasIdxRaw);

// Now do the queue work __ if_then(index, BoolTest::ne, zeroX); {

Node* next_index = _gvn.transform(new SubXNode(index, __

ConX(sizeof(intptr_t)))); Node* log_addr = __ AddP(no_base, buffer, next_index);

// Order, see storeCM.
__ store(__ ctrl(), log_addr, card_adr, T_ADDRESS,

Compile::AliasIdxRaw, MemNode::unordered); __ store(__ ctrl(), index_adr, next_index, TypeX_X->basic_type(), Compile::AliasIdxRaw, MemNode::unordered);

} __ else_(); { __ make_leaf_call(tf, CAST_FROM_FN_PTR(address, SharedRuntime::g1_wb_post), "g1_wb_post", card_adr, __ thread()); } __ end_if(); . . .

The 3 stores StoreCM -> StoreP -> StoreX which mark the card and then push the card address into the dirty queue appear to end up being emitted in that order -- I assume by virtue of the memory links between them (output of StoreCM is the mem input of StoreP, output of StoreP is the mem input of StoreX). That order makes sense if the queue were to be observed concurrently i.e. mark card to ensure write is flagged before it is performed, write value, then decrement index to make value write visible. So, that's ok on x86 where TCO means this is also the order of visibility.

The StoreCM is 'ordered' i.e. it is flagged with mem ordering type = mo_release. However, the latter pair of instructions are 'unordered'. I looked at the G1 code which processes dirty queues and could not make head nor tail (oops, apologies for any undercurrent of a pun) of when it gets run.

So, the question is this: does dirty queue processing only happen in GC threads when mutators cannot be writing them? or is there a need on non-TCO architectures to maintain some sort of consistency in the queue update via by serializing these writes?

Hmm, ok that seems to be two questions. Let's make that a starter for 10 and a bonus for whoever buzzes fastest.

regards,

Andrew Dinn

Senior Principal Software Engineer Red Hat UK Ltd Registered in UK and Wales under Company Registration No. 3798903 Directors: Michael Cunningham (USA), Matt Parson (USA), Charlie Peters (USA), Michael O'Neill (Ireland)

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