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

[Konstantin Osipov via Tarantool-patches <tarantool-patches@dev.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.

Thoughts?

> 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
https://scylladb.com