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

[Vladimir Davydov]

On Mon, Feb 25, 2019 at 07:46:58PM +0300, Konstantin Osipov wrote:
> * Vladimir Davydov <vdavydov.dev at gmail.com> [19/02/24 21:24]:
> > On Sun, Feb 24, 2019 at 08:15:04PM +0300, Konstantin Osipov wrote:
> > > * Vladimir Davydov <vdavydov.dev at gmail.com> [19/02/24 10:01]:
> > > > On Fri, Feb 22, 2019 at 09:37:25PM +0300, Konstantin Osipov wrote:
> > > > > * Kirill Shcherbatov <kshcherbatov at 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.

Okay, pushed the patch that keeps rtree_iterator_pool.