Hi, I have been doing some performance testing with couch and am hoping someone here will be able to help me ascertain if/how I can get higher throughput.
Scenario: I am trying to measure max couch throughput - for these tests im happy with just repeatedly requesting the same document. I have some reasonable boxes to perform these tests - they have dual quad core X5550 CPUs with HyperThreading enabled and 24GB RAM. These boxes have a stock install of oracle enterprise linux 5 on them (which is pretty much RHEL5). The oracle supplied erlang version is R12B5 and I am using couch 1.0.1 built from source. The database is pretty small (just under 100K docs) and I am querying a view that includes some other docs (the request contains include_docs=true) and using jmeter on another identical box to generate the traffic. The total amount of data returned from the request is 1467 bytes. For all of my tests I capture system state using sadc and there is nothing else happening on these boxes. In my initial round of testing I found that I was only getting ~126 requests/s throughput which surprised me somewhat. Looking at the generated graphs from the test run there were plenty of resources to go round - the disk controller was nowhere near busy and neither was the cpu. Before coming here to question my findings I took a 3rd box (same spec) and built couch from the tip of the 1.1.x branch (rev 1040477). After compiling couch and installing it I found that it didn't start up (or log anything useful). After a bit of digging I figured it's probably due to the age of the erlang version being used - I upgraded to OTP R14B and rebuilt couch against it. This gave me a working install again. I got an immediate throughput increase to ~500 requests/s which was nice but the data being collected via sadc still showed that the cpu was at most 20% utilised and the disk controller was doing next to nothing (I assume the OS cache already has the data requested so no trip to disk required?) At this point I started to wonder if jmeter is unable to send in enough requests to stress couch so I started up another jmeter instance on another box and had it also send in requests to couch. What i noticed was that the total throughput didn't increase - it was just split over both jmeter instances. This made me start to think maybe there is something going on in the erlang vm that's stopping me getting higher throughput. Did some digging around and read this: http://erlang.2086793.n4.nabble.com/Some-facts-about-Erlang-and-SMP-td2108770.html Granted the information is a bit stale but that post made me start thinking that maybe I am seeing contention around the run-queue. I see that in R14B I can pass the erlang vm the '+S N:N' flag to control the number of run-queues and how many of them are active. I did a bit of tweaking and ended up getting 700 requests/s by using '+S 16:2". I don't seem to be able to get any more than this though and the system is still not really stressed - CPU is just under 20% and very little disk i/o. Can anyone offer up any advice/suggestions on where to go next? Thanks in advance Huw
