[tor-relays] Overloaded state indicator on relay-search

[David Goulet]

On 27 Sep (14:23:34), Gary C. New via tor-relays wrote:

>  George,
> The referenced support article provides recommendations as to what might be
> causing the overloaded state, but it doesn't provide the metric(s) for how
> Tor decides whether a relay is overloaded. I'm trying to ascertain the
> later.  I would assume the overloaded state metric(S) is/are a maximum
> timeout value and/or reoccurrence value, etc.  By knowing what the
> overloaded state metric is, I can tune my Tor relay just short of it.  Thank
> you for your reply.  Respectfully,

Hi Gary!

I'll try to answer the best I can from what we've have worked on for these
overload metrics.

Essentially, there few places within a Tor relay that we can easily notice an
"overloaded" state. I'll list them and tell you how we decide:

1. Out-Of-Memory invocation

  Tor has its own OOM and it is invoked when 75% of the total memory tor
  thinks it can use is reached. Thus, let say tor thinks it can use 2GB in
  total then at 1.5GB of memory usage, it will start freeing memory. That, is
  considered an overload state.

  Now the real question here is what is the memory "tor thinks" it has.
  Unfortunately, it is not the greatest estimation but that is what it is.
  When tor starts, it will use MaxMemInQueues for that value or will look at
  the total RAM available on the system and apply this algorithm:

    if RAM >= 8GB {
      memory = RAM * 40%
    } else {
      memory = RAM * 75%
    }
    /* Capped. */
    memory = min(memory, 8GB) -> [8GB on 64bit and 2GB on 32bit)
    /* Minimum value. */
    memory = max(250MB, memory)

  Why we picked those numbers, I can't tell you that, these come from the very
  early days of the tor software and I can't tell you why.

  And so to avoid such overload state, clearly run a relay above 2GB of RAM on
  64bit should be the bare minimum in my opinion. 4GB would be much much
  better. In DDoS circumstances, there is a whole lot of memory pressure.

  One keen observer can notice that this approach also has the problem that it
  doesn't shield tor from being called by the OS OOM itself. Reason is that
  because we take the total memory on the system when tor starts, if the
  overall system has many other applications running using RAM, we end up
  eating too much memory and the OS could OOM tor without tor even noticing
  memory pressure. Fortunately, this is not a problem affecting the overload
  status situation.

2. Onionskins processing

  Tor is sadly single threaded _except_ for when the "onion skins" are
  processed that is the cryptographic work that needs to be done on the famous
  "onion layers" in every circuits.

  For that we have a thread pool and outsource all of that work to that pool.
  It can happen that this pool starts dropping work due to back pressure and
  that in turn is an overload state.

  Why this can happen, essentially CPU pressure. If your server is running at
  capacity and it is not only your tor, then this is likely to trigger.

3. DNS Timeout

  This applies only to Exits. If tor starts noticing DNS timeouts, you'll get
  the overload flag. This might not be because your relay is overloaded in
  terms of resources but it signals a problem on the network.

  And DNS timeouts at the Exits are a _huge_ UX problem for tor users and so
  Exit operators really need to be on top of those to help. It is not clear
  with the overload line but at least if an operator notices the overload
  line, it can then investigate DNS timeouts in case there is no resources
  pressure.

4. TCP port exhaustion

  This should be extremely rare though. The idea about this one is that you
  ran out of TCP ports which is a range that is usually, on Linux, 32768-60999
  and so having that many connections would lead to the overload state.

  However, I think (I might be wrong though) that nowadays this range is per
  source IP and not process wide so it would likely have to be deliberate from
  someone to put your relay in that state.


There are two other overload lines that tor relay report:
"overload-ratelimits" and "overload-fd-exhausted" but they are not used yet
for the overload status on Metrics. But you can find them in your relay
descriptor[0] if you are curious.

They are about when your relay reaches its connection global limit too often
and when your relay runs out of file descriptors.

Hope this helps but overall as you can see, a lot of factor can influence
these metrics and so the ideal ideal ideal situation for a tor relay is that
it runs alone on a fairly good machine. Any kinds of pullback from a tor relay
like being overloaded has cascading effects on the network both in terms of UX
but also in terms of load balancing which tor is not yet very good at (but we
are working on hard on making it much better!!).

Cheers!
David

[0]

https://collector.torproject.org/recent/relay-descriptors/server-descriptors/?C=M;O=D

-- 
+7Xz1XCshqTyudrO7K4kGBEl+NghDNbqiTGYZpSOw4U=
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