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Tor sometimes logs entries similar to this:

Sep 27 03:21:32.000 [notice] TLS write overhead: 13%

What does this line mean? Is there a value where this indicates problems?

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See ticket 7707 for why we added that diagnostic log message in. In short, we were worrying that our round-robin bandwidth allocations would spread bytes too thin over all the connections we write out onto, making the overhead from TLS record headers too high a fraction of the overall bandwidth we use.

This log message is telling you how many extra bytes your Tor is using in TLS record headers vs how many bytes it's using for actual Tor cells. Based on the function comment for tls_get_write_overhead_ratio(), it's the "ratio of the bytes that TLS has sent to the bytes that we've told it to send".

Do a git show 452cfaac in a Tor git checkout if you want to see the commit that added this log message.

The ChangeLog stanza is:

  • Add another diagnostic to the heartbeat message: track and log overhead that TLS is adding to the data we write. If this is high, we are sending too little data to SSL_write at a time. Diagnostic for bug 7707.

I just added a pile of questions to ticket 7707, so keep an eye there if Nick answers.

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