[Tarantool-patches] [small] Revert "Free all slabs on region reset"

[Konstantin Osipov]

* Alexander Turenko <alexander.turenko at tarantool.org> [20/01/30 10:27]:
> The Georgy's fix for small keeps linear traversal over allocated slabs
> on region_reset(). Fixed region_reserve() releases one slab at max per
> allocation, so this linear traversal will persist for several
> region_reset() calls in row.

Why do you think region_reset() should traverse over empty slabs?
Empty slabs should always remain at the end of the list, let's
stick to this invariant.

> I think that reserve/reset should give predictable performance and so
> should be constant until region_free() will be called. Ideally it should
> doing something like Georgy made: put one or several unnecessary slabs
> to a slab_cache. Even better would be spread deallocations across
> allocation calls like LuaJIT does: perform more deallocations if there
> are more garbage.

region_free()/region_reset() should be (and is in most cases)
bounded, since it is called as soon as region_used() reaches 128KB.
So yes, it's a list traversal, but it is done only once in every
dozen or so requests and is bounded by a list which is a few
dozens of slabs.

There is no need to complicate it beyond that. It
worked fine for 10 years, after all.


> 
> BTW, #4750 is only about tests and will be closed soon. If you have
> thoughts about gains we can get from changing of region_*() algorithms,
> please, file a separate issue and describe them.
> 

The fix that Georgy made in May was not only about tests. It was
improving region memory usage (and thus indirectly performance),
too.

-- 
Konstantin Osipov, Moscow, Russia