[ntp:questions] Re: stratum

David L. Mills mills at udel.edu
Mon Sep 26 16:41:23 UTC 2005


Terje,

See the PTTI paper at www.eecis.udel.edu/~mills/papers.html. The world 
is a nasty place more horrible than you can imagine and yes, ther are 
tempo terrorists that poll continuously at rates above two per second. 
The NIST servers collectively have over 25 million clients. At one site 
the aggregate rate is in the thousands of packets per second spread over 
three load-balanced servers. Even so, the CPUs won't begin to glow until 
reaching several times that rate.

Dave

Terje Mathisen wrote:

> Richard B. Gilbert wrote:
> 
> 
>>The "maximum" number of clients a server can handle depends on many
>>things.  Perhaps the most  important consideration is network
>>bandwidth.   I've read somewhere that rackety.udel.edu, a stratum one
>>server, run on a Sun IPX (antique hardware (12-15 years old??), serves
>>something like 672 clients and uses a large fraction of a T1 line
> 
> 
> That can't be right!
> 
> I'm almost certain the real number of clients is much higher, maybe an
> order of magnitude or two?
> 
> The results from 'ntpdc -c monlist' is limited by the 600-700 entry
> monitoring array, which is in the right ballpark to come up with your
> 672 figure, but doesn't have to indicate that this is the actual number
> of clients:
> 
> Worst case each client will poll the server every 64 seconds, each
> packet is also about 64 bytes (or a little larger, but this doesn't make
> too much of a difference here), so each client generates about 1
> byte/second of IO (in both directions).
> 
> A T1 line should be able to sustain at least 150-180 KB/sec, giving a
> similar number of clients before the network is saturated.
> 
> Note however that most well-behaved clients will settle down to polling
> much less frequently than every 64 seconds, on my machines most
> stabilize at 1024 second poll intervals, with short-term drops to 256 or
> 512 before returning.
> 
> 
>>(1.5Mbits/second).   Modern hardware, served by a T3 line (45Mb/second
>>and ca. $8000 US/month) should be able to handle forty-five to fifty
>>thousand clients, assuming that the clients are well behaved.
> 
> 
> More.
> 
> Worst-case clients have been seen to poll every second, in which case a
> T3 will saturate at ~80 thousand packets/requests per second.
> 
> Terje




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