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<div class="moz-cite-prefix">On 4/8/19 12:28 PM, Aleksey Shipilev
wrote:<br>
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cite="mid:7ee48121-d4a5-08d7-0f4a-ddf116764e0d@redhat.com">
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<pre class="moz-quote-pre" wrap="">However, you also said in your opening criticism
"I had hard time convincing myself that code is concurrency-safe"
I think that is a more telling complaint. Can you elaborate on why you
found it hard to convince yourself of this? (I know what I think is the
</pre>
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<pre class="moz-quote-pre" wrap="">Because the whole thing in current code is "benign data race" on hash field. Pulling in another
field into race needs careful consideration if it breaks the benignity. It apparently does not, but
the cognitive complexity involved in reading that code makes the minuscule benefit much more
questionable.
-Aleksey
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<br>
Hi Aleksey,<br>
<br>
The reasoning is very similar as with just one field. With one field
(hash) the thread sees either the default value (0) or a non-zero
value calculated either by this thread sometime before or by a
concurrent thread that has already stored it. Regardless of
ordering, the thread either uses the non-zero value or
(re)calculates it (again). The value calculation is deterministic
and uses immutable published state (the array), so it always
calculates the same value for the same object. Idempotence is
guaranteed.<br>
<br>
The same reasoning can be extended to a general case where there are
many fields used for caching of a calculated state from some
immutable published state. The constraint is that the calculation
must be deterministic and must also deterministically choose which
of the many fields used for caching is to be modified. Only one
field may be modified, never more than one. The thread therefore
sees either the default values of all fields or the default values
of all but one field which has been set by either this thread
sometime before or by a concurrent thread. Regardless of ordering,
the thread either uses the state combined from the default values of
all fields but one and a non-default value of a single field or
(re)calculates the non-default value of the single field. The value
calculation is deterministic, uses immutable published state and
deterministically chooses the field to modify, so it always
calculates the same "next" state for the object. Idempotence is
guaranteed.<br>
<br>
Regards, Peter<br>
<br>
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