[ntp:questions] Detecting bufferbloat via ntp?

Dave Täht d at taht.net
Wed Feb 9 00:36:41 UTC 2011


unruh <unruh at wormhole.physics.ubc.ca> writes:

> On 2011-02-08, Rick Jones <rick.jones2 at hp.com> wrote:
>> Dave T??ht <d at taht.net> wrote:
>>> I've been racking my brain trying to come up with a good way of
>>> semi-passively detecting bufferbloat at the datacenter. 
>>
>>> What would wild swings in latency on the order of seconds from a ntp
>>> client register on a ntp server as?
>>
>> Trying to avoid ICMP fast paths?  Once everything is "stable" the
>> polling interval is going to get pretty large (1024 seconds) - watch
>> long enough and I suppose one will see buffer bloat in the stats, but
>> it might take quite a while to "hit." You may need/want to look for it
>> a bit more "actively."
>
> Not at all clear what buffer bloat is supposed to be. This seems to
> implyu that it is buffer leakage-- ie the buffer keeps growing because
> stuff is not properly removed from the buffer. The pointers on the we
> seem to imply that the program assigns buffers which are far too large
> and that therefor the buffers will need to get paged in slowing
> everything down. 

Gettys explains it at about 750 milli-lampsons and a presentation in
about 20 minutes here:

http://mirrors.bufferbloat.net/Talks/BellLabs01192011/murray_hill01192011_Bufferbloat_Talk_Edited_For_brevity.mp3

+

http://mirrors.bufferbloat.net/Talks/BellLabs01192011/110126141155_BufferBloat11.ppt

Some of the periodic behavior he's seeing worries Van Jacobson. See
slides 23-32.

>
> The network buffers in ntp are tiny. The datagram is far less than 1K. 

It is being queued up behind potentially thousands of other FIFO packets
contained in dark, unmanaged buffers between the TCP stack and the edge
gateway. 

> I have no idea what the OP is asking-- is he afraid that the writers of
> ntpd were incompetent and wants to test this particular form of
> incompetence? Has he seen evidence that seems to imply that ntpd suffers
> from bufferbloat? 

No, I am trying to come up with a reasonably passive way to detect
bufferbloat - which is only (currently) observable by doing latency
under load tests on the network edge - in the data center.

>
> If the latter why not tell us the symptoms that make him suspect this. 
> If the former, what makes him think that the programmers screwed up?

I think ntp is great. I also theorize that clocks are wandering more
than we might expect, due to bufferbloat.

>
>>
>> rick jones
>>
>> keeps forgetting if any of the interface MIBs specify an outbound
>> queue length statistic...
>>

-- 
Dave Taht
http://nex-6.taht.net




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