[tarantool-patches] Re: [PATCH 1/2] swim: pool IO tasks

[Konstantin Osipov]

* Vladislav Shpilevoy <v.shpilevoy at tarantool.org> [19/07/08 22:14]:
> > What happens if I create multiple instances of swim library in
> > multiple threads? These instances will try to concurrently access
> > these members without mutexes.
> 
> How is it possible? SWIM works in TX only. You just can't
> create it another thread. It is literally impossible.

My point is, the library should either explicitly prohibit it by
panicking in swim_new/swim_cfg, or should be resilient to it.
Please choose. I'd rather make it not depend on tx or any other
thread in particular, it's easier to test.

> Even if someday it will work in another thread, still all
> the SWIMs will work in one thread. So in future we won't
> allow to scatter SWIMs among multiple threads as well.
> 
> With the same reasoning we would need to make thread-local
> all global on_replace triggers like on_replace_vinyl_deferred_delete
> just in case if in future Vinyl will do DML/DQL in multiple
> threads. But why should we do that now? I do not understand.
> 
> >>
> >>>
> >>> Why not use mempool?
> >>>
> >>>
> >>
> >> Because 1) it is an overkill, 2) I don't want to depend on
> >> slab allocator, 3) it just does not fit this case, according
> >> to mempool description from mempool.h:
> >>
> >>     "Good for allocating tons of small objects of the same size.".
> > 
> > It is also quite decent for allocating many fairly large objects.
> > The key point is that the object is of the same size. You can set
> > up mempool the right slab size, and in this case it will do
> > exactly what you want.
> > 
> 
> And again - 'many' usually won't be the case. We will have 0-2 SWIMs
> in 99% of cases. One internal SWIM for box, and one external created
> by a user. Barely we will have more than 10 cached tasks.
> 
> But ok, as you wish. This place is not as critical for me as thread
> locality of the pool. At least we reuse existing code. Thread locality
> still looks pointless waste of memory for me.

OK, wait a second. I thought you're going to have a single task
instance for each member. Which means a couple of dozen instances
even in a two node cluster. Am I wrong? 

-- 
Konstantin Osipov, Moscow, Russia