[tarantool-patches] Re: [PATCH v3 1/7] memtx: introduce universal iterator_pool

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

* Vladimir Davydov <vdavydov.dev@gmail.com> [19/02/24 21:24]:
> On Sun, Feb 24, 2019 at 08:15:04PM +0300, Konstantin Osipov wrote:
> > * Vladimir Davydov <vdavydov.dev@gmail.com> [19/02/24 10:01]:
> > > On Fri, Feb 22, 2019 at 09:37:25PM +0300, Konstantin Osipov wrote:
> > > > * Kirill Shcherbatov <kshcherbatov@tarantool.org> [19/02/22 19:29]:
> > > > > Memtx uses separate mempools for iterators of different types.
> > > > > Due to the fact that there will be more iterators of different
> > > > > sizes in a series of upcoming changes, let's always allocate the
> > > > > iterator of the largest size.
> > > > 
> > > > If rtree iterator is the one which is largest, let's use a
> > > > separate pool for it. 
> > > > 
> > > > In general mempools are rather cheap. Each mempool takes a slab
> > > > for ~100 objects and uses no slabs if there are no objects (e.g.
> > > > if rtree index is not used, there is no mempool memory for it).
> > > 
> > > But I'd rather prefer to use the same mempool for all kinds of iterator
> > > objects to simplify the code. Take a look at how those mempools are
> > > initialized on demand. IMO it looks ugly. Do we really want to save
> > > those 500 of bytes that much to put up with that complexity?

I don't know much, but a typical SAP R3 which I working on making 
work well with one open source database back in 2003 had ~100k
open client cursors. This could easily entail hundreds of
thousands of server side iterators.

> Regarding the bps tree performance issue. I see nothing wrong about it.
> We've found an issue and we'll surely fix it. There was no point to
> think about such a minor optimization until we actually faced the
> problem. My point is we should strive to write simple and reliable code
> first, and optimize it only if there's a demand, otherwise we risk
> turning the code into unmaintainable mess for no good reason.

I agree with your point, I just disagree that this optimization is
not practical.

-- 
Konstantin Osipov, Moscow, Russia, +7 903 626 22 32
http://tarantool.io - www.twitter.com/kostja_osipov