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From: Konstantin Osipov via Tarantool-patches <tarantool-patches@dev.tarantool.org>
To: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Cc: tarantool-patches@dev.tarantool.org
Subject: Re: [Tarantool-patches] [PATCH v3] wal: introduce limits on simultaneous writes
Date: Tue, 16 Mar 2021 09:45:40 +0300	[thread overview]
Message-ID: <20210316064540.GA39305@starling> (raw)
In-Reply-To: <54666e16-f577-c9f5-e917-f834b7e4b8d5@tarantool.org>

* Vladislav Shpilevoy <v.shpilevoy@tarantool.org> [21/03/16 09:16]:

BTW, one option is to change semantics of "async" to be "best
effort async". That is, if the queue is full, we don't drop the
transaction, we turn it into a waiting one.


> Hi! Thanks for the patch!
> I must admit, it looks kind of ugly :D. The class we have now only remotely
> looks like a semaphore.
> Number of reasons, some of which you already listed in the header file:
> - It is advisory. Can be bypassed easily if you forget to check wouldblock.
>   But not a big issue really. An optional thing like 'try_take' is needed
>   for box.commit({is_async = true}) anyway, not to block the fiber;
> - You can take more amount of the resource than there is. Bearable as well,
>   but still;
> - sem_release() does not wakeup anybody. Quite counter-intuitive;
> - The wouldblock check not only checks the resource being available, but also
>   if there are any waiters. It wouldn't matter for a real semaphore, because
>   it has nothing to do with ordering the waiters in FIFO. It is a detail of
>   the journal which slipped into the general class.
>   But maybe that is the only way to make it fair? Otherwise some fibers
>   could be blocked forever due to starvation.
> The last thing I am not sure is even an issue. Might be a feature.
> The others probably can be fixed if we would rework journal_queue API. For
> instance, not have journal_queue_wait() separated from journal_queue_on_append().
> Then sem_take() could become blocking and obligatory.
> You simply inline everything into journal_write() and journal_write_try_async(),
> and you will see that you can always call take() and block inside of it.
> But I don't know if it is worth doing TBH. It is used in a single place so far.
> This is hard to define fiber_sem API which would be suitable for future usages.
> I would vote for not doing it now and see if we would need the semaphore in the
> future.
> Although the idea about removing journal_queue_wait() might be worth trying.
> It is used either right before journal_queue_on_append(), or in
> journal_queue_flush() which is also right before journal_queue_on_append().
> Up to you. Anyway we need to return to this code for box.commit({is_async})
> feature, which means the hard polishing might be not so useful.

Konstantin Osipov, Moscow, Russia

  reply	other threads:[~2021-03-16  6:45 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-24 19:35 Serge Petrenko via Tarantool-patches
2021-02-24 19:40 ` Serge Petrenko via Tarantool-patches
2021-02-25 13:05 ` Konstantin Osipov via Tarantool-patches
2021-02-26  0:57   ` Vladislav Shpilevoy via Tarantool-patches
2021-02-26  7:18     ` Konstantin Osipov via Tarantool-patches
2021-02-26 20:23       ` Vladislav Shpilevoy via Tarantool-patches
2021-02-26 21:20         ` Konstantin Osipov via Tarantool-patches
2021-02-26 22:44           ` Vladislav Shpilevoy via Tarantool-patches
2021-02-27 13:27             ` Konstantin Osipov via Tarantool-patches
2021-03-01 19:15   ` Serge Petrenko via Tarantool-patches
2021-03-01 21:46     ` Konstantin Osipov via Tarantool-patches
2021-02-26  0:56 ` Vladislav Shpilevoy via Tarantool-patches
2021-03-01 19:08   ` Serge Petrenko via Tarantool-patches
2021-03-01 22:05     ` Vladislav Shpilevoy via Tarantool-patches
2021-03-02 17:51       ` Serge Petrenko via Tarantool-patches
2021-03-03 20:59         ` Vladislav Shpilevoy via Tarantool-patches
2021-03-09 15:10           ` Serge Petrenko via Tarantool-patches
2021-03-09 19:49 ` Vladislav Shpilevoy via Tarantool-patches
2021-03-10  8:18   ` Konstantin Osipov via Tarantool-patches
2021-03-12 17:10     ` Serge Petrenko via Tarantool-patches
2021-03-13 19:14       ` Konstantin Osipov via Tarantool-patches
2021-03-15 23:42       ` Vladislav Shpilevoy via Tarantool-patches
2021-03-16  6:45         ` Konstantin Osipov via Tarantool-patches [this message]
2021-03-16 20:27           ` Vladislav Shpilevoy via Tarantool-patches
2021-03-16 10:19         ` Serge Petrenko via Tarantool-patches
2021-03-16 20:48           ` Vladislav Shpilevoy via Tarantool-patches
2021-03-17 12:14             ` Serge Petrenko via Tarantool-patches
2021-03-17 21:02           ` Vladislav Shpilevoy via Tarantool-patches
2021-03-19 11:32             ` Serge Petrenko via Tarantool-patches
2021-03-19 15:36 ` Kirill Yukhin via Tarantool-patches

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