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. 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. Here is a munin graph clearly showing it: http://cl.ly/image/3V2A1J261o30 Before midnight, the SSD (Samsung 840 EVO 1TB) is on a PCIe/SATA adapter (Apricorn Velocity Solo x1). After 1:00 it moved to the builtin SAS/SATA enclosures driven by the RAID card. The server is a Dell R710 with 48GB RAM running ubuntu server 14.04. I'm using mapnik 2.3.x with async postgis queries enabled. -- Christian Quest - OpenStreetMap France
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