On 3/7/19 4:01 PM, Robert Haas wrote: > On Thu, Mar 7, 2019 at 9:49 AM Tomas Vondra > <tomas.von...@2ndquadrant.com> wrote: >> I don't think this shows any regression, but perhaps we should do a >> microbenchmark isolating the syscache entirely? > > Well, if we need the LRU list, then yeah I think a microbenchmark > would be a good idea to make sure we really understand what the impact > of that is going to be. But if we don't need it and can just remove > it then we don't. > >> What I had in mind is more along these lines: >> >> (a) track number of active syscache entries (increment when adding a new >> one, decrement when evicting one) >> >> (b) track peak number of active syscache entries >> >> (c) after clock-sweep, if (peak > K*active) where K=2 or K=4 or so, do a >> memory context swap, i.e. create a new context, copy active entries over >> and destroy the old one >> >> That would at least free() the memory. Of course, the syscache entries >> may have different sizes, so tracking just numbers of entries is just an >> approximation. But I think it'd be enough. > > Yeah, that could be done. I'm not sure how expensive it would be, and > I'm also not sure how much more effective it would be than what's > currently proposed in terms of actually freeing memory. If you free > enough dead syscache entries, you might manage to give some memory > back to the OS: after all, there may be some locality there. And even > if you don't, you'll at least prevent further growth, which might be > good enough. >
I have my doubts about that happening in practice. It might happen for some workloads, but I think the locality is rather unpredictable. > We could consider doing some version of what has been proposed here > and the thing you're proposing here could later be implemented on top > of that. I mean, evicting entries at all is a prerequisite to > copy-and-compact. > Sure. I'm not saying the patch must do this to make it committable. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
