[Lldb-commits] [PATCH] D62931: [lldb-server] Add setting to force 'g' packet use

[Pavel Labath via Phabricator via lldb-commits]

labath added reviewers: clayborg, jasonmolenda.
labath added a comment.

Personally, I'm wondering whether we even need a setting for this. Running the speed test command locally (i.e. the lowest latency we can possibly get), I get results like:

  (lldb) process plugin packet speed-test --count 100000 --max-receive 2048
  Testing sending 100000 packets of various sizes:
  ...
  qSpeedTest(send=      4, recv=      0) in 0.788656771 s for 126797.88 packets/s (0.007886 ms per packet) with standard deviation of 0.001604 ms
  qSpeedTest(send=      4, recv=      4) in 0.770005465 s for 129869.21 packets/s (0.007700 ms per packet) with standard deviation of 0.002120 ms
  qSpeedTest(send=      4, recv=      8) in 0.895460367 s for 111674.40 packets/s (0.008954 ms per packet) with standard deviation of 0.002048 ms
  qSpeedTest(send=      4, recv=     16) in 0.910367966 s for 109845.70 packets/s (0.009103 ms per packet) with standard deviation of 0.001886 ms
  qSpeedTest(send=      4, recv=     32) in 0.889442623 s for 112429.96 packets/s (0.008894 ms per packet) with standard deviation of 0.001705 ms
  qSpeedTest(send=      4, recv=     64) in 0.945124865 s for 105806.12 packets/s (0.009451 ms per packet) with standard deviation of 0.002349 ms
  qSpeedTest(send=      4, recv=    128) in 0.759995580 s for 131579.72 packets/s (0.007599 ms per packet) with standard deviation of 0.002971 ms
  qSpeedTest(send=      4, recv=    256) in 0.847535312 s for 117989.19 packets/s (0.008475 ms per packet) with standard deviation of 0.002629 ms
  qSpeedTest(send=      4, recv=    512) in 1.022377729 s for  97811.21 packets/s (0.010223 ms per packet) with standard deviation of 0.003248 ms
  qSpeedTest(send=      4, recv=   1024) in 1.436389208 s for  69619.02 packets/s (0.014363 ms per packet) with standard deviation of 0.000688 ms
  qSpeedTest(send=      4, recv=   2048) in 1.910557270 s for  52340.75 packets/s (0.019105 ms per packet) with standard deviation of 0.001194 ms
  ...

Now, if we assume that the `p` resonse is about 16 bytes long, and `g` response is 2048, we find that the `g` packet is worth approximately two `p` packets. And this is a local link, where it pretty much doesn't matter what we do, as we're pumping 50k packets per second even in the slowest case. On links with a non-negligible latency, the difference between the two packets should be even smaller (though it might be nice to verify that).

Combining this with the fact that we normally expedite all general purpose registers (and so there won't be *any* packets if one is only reading those), I would say that enabling this unconditionally is a pretty safe thing to do.

Greg, Jason, what do you think?


Repository:
  rLLDB LLDB

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  https://reviews.llvm.org/D62931/new/

https://reviews.llvm.org/D62931