[tor-bugs] #24737 [Core Tor/Tor]: Recommend a MaxMemInQueues value in the Tor man page

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#24737: Recommend a MaxMemInQueues value in the Tor man page
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 Reporter:  starlight       |          Owner:  (none)
     Type:  defect          |         Status:  new
 Priority:  Medium          |      Milestone:  Tor: 0.3.2.x-final
Component:  Core Tor/Tor    |        Version:
 Severity:  Normal          |     Resolution:
 Keywords:  doc, tor-relay  |  Actual Points:
Parent ID:                  |         Points:  0.5
 Reviewer:                  |        Sponsor:
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Comment (by teor):

 Replying to [comment:5 teor]:
 > Moved conversation from #22255.
 >
 > Replying to [ticket:22255#comment:80 starlight]:
 > > Replying to [ticket:22255#comment:79 teor]:
 > > > I'd still like to see someone repeat this analysis with 0.3.2.8-rc,
 and post the results to #24737.
 > > > It's going to be hard for us to close that ticket without any idea
 of the effect of our changes.
 > >
 > > I'm not willing to run a newer version till one is declared LTS, but
 can say that even when my relay is not under attack memory consumption
 goes to 1.5G with the 1G max queue setting.  Seems to me the 2x max queues
 memory consumption is a function of the overheads associated with tor
 daemon queues and related processing, including malloc slack space.
 >
 > Saying 2x is a useful guide, but I think we can do better. Because I see
 very different behaviour on systems with a lot more RAM.
 >
 > This is how the overheads work on my 0.3.0 relay with 8 GB per tor
 instance, and a high MaxMemInQueues:
 > * 512 MB per instance with no circuits
 > * 256 - 512 MB extra per instance with relay circuits
 > * 256 - 512 MB extra per instance with exit streams
 > The RAM usage will occasionally spike to a few gigabytes, but I've never
 seen it all used.
 >
 > So I think we should document the following RAM usage and MaxMemInQueues
 settings:
 > * Relays: minimum 768 MB, set MaxMemInQueues to (RAM per instance - 512
 MB)*N
 > * Exits: minimum 1GB, set MaxMemInQueues to (RAM per instance - 768
 MB)*N
 >
 > For all versions without the destroy cell patch (0.3.2.7-rc and all
 current versions as of 1 January 2018), N should be 0.5 or lower. It's
 reasonable to expect destroy cell queues and other objects to take up
 approximately the same amount of RAM as the queues.
 >
 > For all versions with the destroy cell patch (0.3.2.8-rc and all
 versions released after 1 January 2018), N should be 0.75 or lower. It's
 reasonable to expect destroy cell queues and other objects to take up a
 third of the queue RAM.
 >
 > Now we just have to turn this into a man page patch and wiki entry.

 Here's some advice I've just given some relay operators:

 If you have 4 Tor Exits, a 1 Gbps connection, and this much RAM, use this
 setting:
 8 GB RAM -> MaxMemInQueues 256 MB
 16 GB RAM -> MaxMemInQueues 1 GB
 32 GB RAM -> MaxMemInQueues 2 GB

 I think this is the right level of detail for a man page.

 We could probably afford 3 GB with 32 GB of RAM, but there are other
 issues:
 * do we really benefit from buffering more than a minute of traffic?
 * how much extra CPU load do we get if we set MaxMemInQueues too high?
 * how low does MaxMemInQueues need to be to resist a sniper attack?

 I also opened #24782 so we change the default in Tor itself.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24737#comment:6>
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