SintMaarten network
In October 2009 the SintMaarten cluster was commissioned. This cluster is based on HP blades. Soon after commissioning a serious performance issue was reported:
[BG-NLT1-Support] #287: bad gridftp transfer rate - smrt wns
This page is the result of the analysis of this performance issue.
Problem report
The performance issue reported was seen when copying a file from the Nikhef storage system to a SintMaarten worker node. Transfer speeds at first were OK but dropped to very low levels after about 120 Mb of data, eventually causing timeouts in the lcg-cp command used. Copying the exact same file from the exact same storage element to a slightly older worker node did not experience this problem:
=== wn-smrt-006 (Bad!) === # lcg-cp --vo atlas -v srm://.... file://..... [snip] # streams: 1 62914560 bytes 1279.98 KB/sec avg 512.00 KB/sec inst
vs
=== wn-val-066 (Good!) === # lcg-cp --vo atlas -v srm://.... file://..... [snip] # streams: 1 1672478720 bytes 68053.21 KB/sec avg 70142.84 KB/sec inst
Analysis
At first it was thought that the lcg-cp command itself was causing the error:
- when copying the file using lcg-cp the command timed out after several minutes
- when copying the exact same file using globus-url-copy the command finished in less than a minute
However when using
globus-url-copy -nodcau
the command also timed out. It is worth noting that
- -nodcau means 'no data channel authentication' ; it makes file transfers less secure but it works better when firewalls are in place
- lcg-cp also disables data channel authentication
The next step was to rule out the grid middleware altogether: by using netcat (nc) to transfer a file we also managed to bring the network speed to a crawl:
on wn-smrt-011.farm.nikhef.nl: # nc -l 20009 > /tmp/bigfile on hooi-ei-09: # cat bigfile | nc wn-smrt-011.farm.nikhef.nl 20009
Transfer speeds are initially OK but after roughly 100 Mb transfer speeds drop to about 200 KB/s. A very useful tool to see this is PipeViewer
# cat bigfile | pv -c | nc wn-smrt-011.farm.nikhef.nl 20009
So now we know it's not the (grid) middleware but more likely a networking issue. The hooi-ei-09 storage element is part of a DPM disk pool. It is a Sun XFire ('Thumper') with 3 gigabit controllers which are bonded using 802.3ad into a single interface using a 3Com switch. Each connection to such a configuration will use only a single network controller, hence a single globus-url-copy should max out at roughly 110 MB/s. The SintMaarten worker nodes have dual Broadcom NetXtreme II 10GE adapters:
# grep Broadcom /var/log/dmesg eth0: Broadcom NetXtreme II BCM57711E XGb (A0) PCI-E x4 5GHz (Gen2) found at mem fb000000, IRQ 114, node addr 0017a4770028 eth1: Broadcom NetXtreme II BCM57711E XGb (A0) PCI-E x4 5GHz (Gen2) found at mem fa000000, IRQ 122, node addr 0017a477002a
of which only the first is connected to an Arista switch. The connection speed is 1 Gbps as can be seen using ethtool eth0:
[root@wn-smrt-011 ~]# ethtool eth0 Settings for eth0: Supported ports: [ FIBRE ] Supported link modes: 1000baseT/Full 2500baseX/Full Supports auto-negotiation: Yes Advertised link modes: 1000baseT/Full 2500baseX/Full 10000baseT/Full Advertised auto-negotiation: Yes Speed: 1000Mb/s Duplex: Full Port: FIBRE PHYAD: 1 Transceiver: internal Auto-negotiation: on Supports Wake-on: g Wake-on: g Current message level: 0x00000000 (0) Link detected: yes [root@wn-smrt-011 ~]# ethtool eth1 | grep Link Link detected: no