From: Lars Schneider <larsxschneider@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: Eric Wong <e@80x24.org>, git@vger.kernel.org
Subject: Re: RFC: Enable delayed responses to Git clean/smudge filter requests
Date: Wed, 16 Nov 2016 10:53:17 +0100 [thread overview]
Message-ID: <17709AFF-3C2D-4EC0-97DC-BD750F514D0B@gmail.com> (raw)
In-Reply-To: <xmqqvavotych.fsf@gitster.mtv.corp.google.com>
On 15 Nov 2016, at 19:03, Junio C Hamano <gitster@pobox.com> wrote:
> Lars Schneider <larsxschneider@gmail.com> 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
next prev parent reply other threads:[~2016-11-16 9:53 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-14 21:09 RFC: Enable delayed responses to Git clean/smudge filter requests Lars Schneider
2016-11-15 1:03 ` Eric Wong
2016-11-15 14:29 ` Lars Schneider
2016-11-15 18:03 ` Junio C Hamano
2016-11-16 9:53 ` Lars Schneider [this message]
2016-11-16 18:15 ` Junio C Hamano
2016-11-16 18:47 ` Lars Schneider
2016-11-16 19:19 ` Junio C Hamano
2016-11-16 22:41 ` Jakub Narębski
2016-11-16 23:46 ` Junio C Hamano
2016-11-17 9:19 ` Lars Schneider
2016-11-24 15:45 ` Lars Schneider
2016-11-28 21:48 ` Junio C Hamano
2016-11-15 18:27 ` Eric Wong
2017-01-09 20:44 ` Stefan Beller
2017-01-11 12:57 ` Lars Schneider
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