Sometimes Less really is More

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Today I made some computers go faster by taking out some of their memory and halving the number of CPUs in each one.

No seriously I did. There were three VMware machines on a dual core 3G Xeon running ESX that we couldn't coax above 150 MHz if there was activity on the NICs, no matter what we fiddled with. Bizarrely an almost identical (virtual) machine on the same (physical) box ran fine. We couldn't find anything different enough between them to warrant this massive difference.

In desparation VMware support in Cork suggested we try cutting the allocated memory and for good measure reducing them to one core each. Amazingly this punitive measure worked; performance leapt by an order of magnitude. Developers were jubilant, managers sighed with relief. Network engineers were mostly apologised to.

The ESX hypervisor will seemingly starve itself of RAM in some cases unless you leave a portion of RAM free, although exactly how much you need to be on the safe side we have yet to determine. For some reason this caused the 'lucky' (virtual) machine to be scheduled to run with far greater likelihood than the other three when the nic was busy. This could well be (and I'm guessing here) an artefact of the way ESX gathers entropy - in lieu of anything better a lot of RNGs take their cues from packet arrival times. It's probable that the number of CPUs is a bit of a red herring - but apparently there are scheduling weirdnesses that can kick in where some of the VMs begin core-squatting (for want of a better term - though the behaviour of taking a resource in order to do nothing with it fits more landlords than real-life squatters).

Coincidentally I learned a wonderful chinese idiom yesterday pronounced roughly "joe joe gwee yee" meaning literally "nine nine return-to one"; which basically means 'integer overflow - carry the one', and is used colloquially to express the idea that taking too much is as bad as never having enough (which is important to much of Chinese philosophy). In the west I suppose we have the game of 'chicken' as a similar cultural reference.

Like most 21st century techies I'm reasonably confident these days working in the bizarro world of virtual hardware, but this was the most counter-intuitive result so far.

21:49 01 Feb 2008 /articles/systems/ less_is_more

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