[tor-relays] Negative impact of MaxOnionsPending 250?

[Scott Bennett]

     On Thu, 26 Jul 2012 17:11:17 +0200 Moritz Bartl <moritz at torservers.net>
wrote:
>We sometimes see "Your computer is too slow to handle this many circuit
>creation requests!" on our servers. Scott Bennett suggested to set
>MaxOnionsPending to 250 instead of the default of 100, which at least
>makes the warnings disappear. [1]
>
>I am thinking about modifying out torrc template we use as as basis for
>all servers. What is the negative impact of setting MaxOnionsPending to
>250? Should we instead reduce the bandwidth as suggested in the warning?

     I don't think so.  To the best of my recollection, I've only seen
this message in recent versions of tor when it appeared that my node had
been chosen for one or more high-volume hidden service operations, where
the traffic volume in B/s was still fairly low, yet the number of connections
had suddenly climbed to an unusually high level.  The data throughput
capacity vastly exceeded the actual traffic, but the onionskin-decoding
load was sometimes, but probably not continually, very high until the hidden
service(s) either shut down or changed their advertised information in
the hidden service directory.  To lower my node's RelayBandwidth* values
would most likely have little impact on the bottleneck.  Raising
MaxOnionsPending adds a little slack in the queue to handle the occasions
where that activity is temporarily backlogged prevents dropped connections
and allows operations to proceed normally with only minimal extra delays.
Consider a delay of probably no more than a second at the outside and
usually less versus the delay of building a new circuit to a hidden service
due to a dropped connection.
>
>[1] https://lists.torproject.org/pipermail/tor-relays/2012-May/001352.html


                                  Scott Bennett, Comm. ASMELG, CFIAG
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