On 15 Nov 2016, at 19:03, Junio C Hamano <gitster@xxxxxxxxx> wrote:
> Lars Schneider <larsxschneider@xxxxxxxxx> writes:
>
>>> The filter itself would need to be aware of parallelism
>>> if it lives for multiple objects, right?
>>
>> Correct. This way Git doesn't need to deal with threading...
>
> I think you need to be careful about three things (at least; there
> may be more):
>
> * Codepaths that check out multiple cache entries do rely on the
> order of checkout. We checkout removals first to make room so
> that creation of a path X can succeed if an existing path X/Y
> that used to want to see X as a directory can succeed (see the
> use of checkout_entry() by "git checkout", which does have two
> separate loops to explicitly guarantee this), for example. I
> think "remove all and then create" you do not specifically have
> to worry about with the proposed change, but you may need to
> inspect and verify there aren't other kind of order dependency.
OK
> * Done naively, it will lead to unmaintainable code, like this:
>
> + struct list_of_cache_entries *list = ...;
> for (i = 0; i < active_nr; i++)
> - checkout_entry(active_cache[i], state, NULL);
> + if (checkout_entry(active_cache[i], state, NULL) == DELAYED)
> + add_cache_to_queue(&list, active_cache[i]);
> + while (list) {
> + wait_for_checkout_to_finish(*list);
> + list = list->next;
> + }
>
> I do not think we want to see such a rewrite all over the
> codepaths. It might be OK to add such a "these entries are known
> to be delayed" list in struct checkout so that the above becomes
> more like this:
>
> for (i = 0; i < active_nr; i++)
> checkout_entry(active_cache[i], state, NULL);
> + checkout_entry_finish(state);
>
> That is, addition of a single "some of the checkout_entry() calls
> done so far might have been lazy, and I'll give them a chance to
> clean up" might be palatable. Anything more than that on the
> caller side is not.
I haven't thought hard about the implementation, yet, but I'll try
to stick to your suggestion and change as less code as possible on
the caller sides.
> * You'd need to rein in the maximum parallelism somehow, as you do
> not want to see hundreds of competing filter processes starting
> only to tell the main loop over an index with hundreds of entries
> that they are delayed checkouts.
I intend to implement this feature only for the new long running filter
process protocol. OK with you?
Thanks,
Lars