Re: [Tarantool-patches] [PATCH] small: unite the oscillation cache of all mempools

[Konstantin Osipov <kostja.osipov@gmail.com>]

* Kirill Yukhin <kyukhin@tarantool.org> [20/01/29 13:30]:
> On 29 янв 04:48, Maksim Kulis wrote:
> > The task is to unite the oscillation cache of all mempools.
> > In the function slab_put_with_order() we check if there is any 
> > slab of the largest size except the current one, in order to 
> > avoid oscillation. 
> > https://github.com/tarantool/small/blob/master/small/slab_cache.c#L349
> 
> Thanks a lot for your patch. Since the change is performance
> related, before we start off review process, please provide a
> testcase (for tarantool or whatever) which will clearly show
> that there're cases where performance is actually increased.
> 
> We won't accept "nice looking" perf patches.

It's not about performance :)

The patch reduces memory fragmentation quite a bit on small-arena
systems. Imagine a 1G system with slab_alloc_factor =1.06
(default). It can easily have up to 50-100 mempools in slab arena,
each holding up one mslab, up to 100-400MB in total. 

The whole point of holding up to such mslab is to avoid mmap()
system call (which is costly) in cases like 

for (i = 1..10000)
    mslab_alloc()
    mslab_free()

This is allocation scenario is called "oscilation".

There is no point in it, because there is already a cached mslab in
slab_cache, and it will be used just fine.

Maksim should update the CS comment.

-- 
Konstantin Osipov, Moscow, Russia