Just coming back from spring break, I read the LDM performance
> This morning, I was benchmarking file transfers using LDM vs copying the
> same file with 'scp'. On the two files I checked, (1237KB and
> 2446KB),LDM took 3 times as long to send the file compared to scp.
> Actually, I do want to use this feature of pqact, but I don't think I
> can afford the overhead of using LDM.
Just as background, the LDM was designed to optimize the delivery of a
large number of small data products, but it was later extended to
deliver large products as well, by splitting them into smaller
segments. Each small product or segment of a larger product is
delivered by a remote procedure call (RPC) that requires a round-trip
If you have control of both upstream and downstream LDMs, you can tune
the protocol for large products by changing the size of segments into
which larger products are split. But you may not have as much control
as you want because TCP is still used to implement the RPC calls, and
it also splits large messages into smaller packets for reliable
delivery. Nevertheless, you could try increasing the maximum size of
a chunk of data in the LDM by changing the definition of DBUFMAX in
#define DBUFMAX 16384
Be warned that if you do this, you have changed the protocol, so don't
expect your changed LDMs to interoperate with other LDMs.
The average size of data products delivered over the IDD has been
increasing over the last few years, but that size is still smaller
than 16384 bytes:
Year prod prods prods mbytes kbytes
size /hr /sec /hr /sec
1995 4971 6357 1.8 32 9
1996 4486 9368 2.6 42 12
1997 5742 9484 2.6 55 15
1998 9680 15507 4.3 157 44
1999 10328 33432 9.3 345 96
2000 11070 42742 11.9 484 134
2001 12505 34629 9.6 438 122
2002 13926 45909 12.8 641 178
That table came from the longterm average of what the motherlode
server (and its predecessor thelma) deliver to a downstream site.
I would be interested in ftp or scp performance at copying a large
number of small products over a long period of time. The LDM can
deliver over 50000 products per hour comprising over 1 Gbyte/hour to
multiple downstream sites for extended periods. In LANs where RPC
latency is negligible, it can deliver up to 2000 products/second.
If the LDMs delivery performance for large products becomes more of an
issue, we may have to use different mechanisms for large products to
achieve something closer to optimum, but so far the number of products
has been more of a problem than their size ...
Russ Rew UCAR Unidata Program