21 February 2013

BNL rate improvements in practice

Thanks to the recent investigative work by Alessandra Forti, the TCP tuning settings on the UKI-SOUTHGRID-OX-HEP DPM disk pool servers have been changed to increase the default sizes of the IPv4 windows to a very large 1MB from the much smaller previous value of ~64KB (which had been taken from the fasterdata.es.net recommendations). The change made an immediate improvement in the results of our artificial testing, but a day later we can now see the pay off in speed of real transfers:

Transfers from Oxford to BNL are the red line that jumps from virtually zero straight up to about 40MB/s, making BNL now one of our fastest destination sites.

We're still not sure what the underlying cause of the apparent problem with automatic window scaling for multi-stream gridftp transfers is, but it appears safe to say that this change has completely removed the practical effects.

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