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From: Serge Petrenko via Tarantool-patches <tarantool-patches@dev.tarantool.org>
To: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>,
	Cyrill Gorcunov <gorcunov@gmail.com>
Cc: tml <tarantool-patches@dev.tarantool.org>
Subject: Re: [Tarantool-patches] [PATCH v23 3/3] test: add gh-6036-qsync-order test
Date: Fri, 22 Oct 2021 09:36:23 +0300	[thread overview]
Message-ID: <79f80140-8c2f-4c34-10ae-186982db5ef1@tarantool.org> (raw)
In-Reply-To: <b5b13af5-a8e2-06dd-7fb8-7d88b242a934@tarantool.org>

22.10.2021 01:06, Vladislav Shpilevoy пишет:
>>>> Actually you do need to count writes here.
>>>> The wait_cond for ERRINJ_WAL_WRITE_COUNT == write_cnt + 3
>>>> is needed to make sure you receive (and thus try to process)
>>>> insert {3} **before** the replica is re-enabled.
>>>> Otherwise we can't be sure that the test is correct. You may simply
>>>> perform a select before insert{3} has reached the replica.
>>> You know, I spent a few hours trying to pass the test waiting for
>>> ERRINJ_WAL_WRITE_COUNT == write_cnt + 3 and finally realized that
>>> it seems that is what happens: the replica1 is not longer a leader
>>> and when this record reach our replica3 node we NOPify it then
>>> we run
>>> apply_row
>>>     if (request.type == IPROTO_NOP)
>>>       return process_nop()
>>> thus this record even not reaching the journal at all and that is
>>> why waiting for write_cnt + 3 lasts forever. If only I didn't miss
>>> something obvious.
>> Unfortunately, this is not the case. A NOP entry still reaches WAL.
>> That's why we need NOP entries: they reside in WAL but do nothing.
>> That's for vclock bump sake. Otherwise we could skip such entries
>> completely, without nopifying them.
>> So, even if the entry is nopified, it would enter WAL sooner or later.
>> I just realised what the problem is: the entry is waiting on a limbo latch
>> inside the NOPify procedure. That's why it never reaches the journal
>> (until we re-enable replica3, at least).
>> I don't know how to wait for this entry's arrival then.
>> The current test version looks OK to me.
>> Vlad, do you have any ideas here?
> I think it might worth adding an errinj for the number of blocked
> fibers waiting on the limbo latch. Could even expose that to box.info.qsync,
> seems like useful info. Would help to measure contention.

This might be useful, indeed.
Cyrill, let's implement `box.info.synchro.queue.waiters` then and use it 
in the test.
Or any other suitable name if you guys come up with one.

Serge Petrenko

  reply	other threads:[~2021-10-22  6:36 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-14 21:56 [Tarantool-patches] [PATCH v23 0/3] qsync: implement packet filtering (part 1) Cyrill Gorcunov via Tarantool-patches
2021-10-14 21:56 ` [Tarantool-patches] [PATCH v23 1/3] latch: add latch_is_locked helper Cyrill Gorcunov via Tarantool-patches
2021-10-14 21:56 ` [Tarantool-patches] [PATCH v23 2/3] qsync: order access to the limbo terms Cyrill Gorcunov via Tarantool-patches
2021-10-21 22:06   ` Vladislav Shpilevoy via Tarantool-patches
2021-10-14 21:56 ` [Tarantool-patches] [PATCH v23 3/3] test: add gh-6036-qsync-order test Cyrill Gorcunov via Tarantool-patches
2021-10-19 15:09   ` Serge Petrenko via Tarantool-patches
2021-10-19 22:26     ` Cyrill Gorcunov via Tarantool-patches
2021-10-20  6:35       ` Serge Petrenko via Tarantool-patches
2021-10-21 22:06         ` Vladislav Shpilevoy via Tarantool-patches
2021-10-22  6:36           ` Serge Petrenko via Tarantool-patches [this message]
2021-10-21 22:06   ` Vladislav Shpilevoy via Tarantool-patches
2021-10-22 22:03     ` Cyrill Gorcunov via Tarantool-patches
2021-10-24 15:39       ` Vladislav Shpilevoy via Tarantool-patches
2021-10-24 16:01         ` Cyrill Gorcunov via Tarantool-patches

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