[OSM-dev] Tuning render_list: beware of hyperthreading and RAID cards latency...

Paul Norman penorman at mac.com
Thu Jul 17 15:03:38 UTC 2014

On 2014-07-17 6:39 AM, Christian Quest wrote:
> I'm currently doing some rendering benchmark to tune a new tile server 
> we're about to put online.
> A strange thing I've found last night: render_list with 16 thread 
> takes more time than with 8 .
> The server has 2 quad-core Xeon (X5570), this means 8 physical cores. 
> It looks like hyperthreading adds additionnal context switches or 
> something similar.
> With "-z 9 -Z 9 -n 16 -l 32" it takes 3016s, and with "-z 9 -Z 9 -n 8 
> -l 12" 2647s, 10% less.
> I'll disable HT on my next run in the server bios and see if it is 
> consistent.

Is there a chance you're running into IO bottlenecks?

> Another thing I found... do not put your SSD behind a RAID card.
> The RAID card (Dell Perc 6i) add too much latency.
> On the same render_list, the iowait threads are increasing a lot and 
> iops decreases a lot.
The Perc 6 is an older lower-end RAID card using a LSI 10xx chip. It 
doesn't surprise me it doesn't work ideally with SSDs. I'm also not sure 
if it supports all the SATA commands a SSD uses, which can be a serious 
issue for consistency, the bane of SSDs.

A more modern card would probably perform better, but if you're not 
RAIDing SSDs, you should really be using any card you have in HBA mode, 
unless you have a battery-backed cache and can therefor tune write 
barriers and related settings better.
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