[Tarantool-patches] [PATCH v3] wal: introduce limits on simultaneous writes

[Serge Petrenko]

16.03.2021 02:42, Vladislav Shpilevoy пишет:
> Hi! Thanks for the patch!

Thanks for your answer!

>
> 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.

Let's drop the semaphore thing then. I removed the commit introducing it and
pushed the patch on a new branch: sp/gh-5536-replica-oom-no-sem

-- 
Serge Petrenko