[tor-relays] Tor relay occasionally maxing out CPU usage

[William Kane]

Dear Alexander,

I have added 'Log [dirserv]info notice stdout' to my configuration and
will be monitoring the system closely.

Tor was also upgraded to version 0.4.3.5, and the linux kernel was
upgraded to version 5.6.13 but I do not think this will change
anything.

Expect a follow-up within the next 12 hours.

William

2020-05-18 1:40 GMT, Alexander Færøy <ahf at torproject.org>:
> Hello,
>
> On 2020/05/17 18:20, William Kane wrote:
>> Occasionally, the CPU usage hit's 100%, and the maximum throughput
>> drops down to around 16 Mbps from it's usual 80 Mbps. This happens
>> randomly and not a fixed intervals which makes it pretty hard to
>> profile.
>
> One of the subsystem's that I can think of that could potentially lead
> to the problem that you are describing is our "consensus diff"
> subsystem. The consensus diff subsystem is responsible for turning
> consensus documents into these patch(1)-like diffs that clients can
> fetch without the need to transfer the whole consensus for each minor
> change.
>
> The subsystem also takes care of compression, which includes LZMA, which
> is a beast when it comes to burning CPU cycles.
>
>> No abnormal entries in the log files.
>
> I suspect you're logging at `notice` log-level, which is the reasonable
> thing to do. We need to log at slightly higher granularity to discover
> the problem here.
>
> Could I get you to add `Log [dirserv]info notice syslog` to your
> `torrc`? This line makes Tor log everything at notice log-level (the
> default), to the system logger, except for the directory server
> subsystem, which will be logged at `info` log-level instead. The code
> responsible for generating consensus diffs uses the `dirserv` for
> logging purposes.
>
> If the CPU spike happens right after a log message that says something
> in the line of "The most recent XXX consensus is valid-after XXX. We
> have diffs to this consensus for XXX/XXX older XXX consensuses.
> Generating diffs for the other XXX." then I think we have our winner.
>
> Please remember to remove the `info` log-level when the experiment is
> over :-)
>
> I'm curious what you figure out here. Let me know if you need any help.
>
> All the best,
> Alex.
>
> --
> Alexander Færøy
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