Log Compression instead of Chatty in Android S

The problem

  • Log buffer space is precious, but suffers from the tragedy of the commons
  • Log spam fills the buffers making them less useful in logcat/bugreports
  • “Spam” is often in the eye of the beholder: which messages are important depends on what you’re trying to debug

The idea

  • Chatty isn’t helping as much as we’d hoped, and is surprisingly expensive
  • Compress logs to make more efficient use of the buffer
  • Address the root cause of log spam at its source:
    • Do not hide log spam at runtime, which de-incentivize fixes
    • Add presubmit coverage similar to SELinux violations to keep log spam down

Chatty in Theory

  • Delete messages classified as spam to extend the range of logs from other sources
  • “Spam” defined as:
    • Logs from UIDs whose logs consume over 12.5% of a log buffer
    • Back-to-back exact duplicate messages

Chatty in Practice

  • Developer confusion about missing and de-duplicated logs
  • Lowered incentive to fix the root cause of bad logging behavior
  • High CPU overhead
  • Memory usage greatly exceeds configured buffer size
  • Only marginal increase in log range

Log Compression in Theory

  • Store many more logs in the same log buffer size => better for diagnosis
  • Memory usage stays below configured log size => better system health
  • No gaps in logs, no de-duplicated logs => no developer confusion
  • No hiding bad behavior => increased accountability/incentive to fix root causes

Log Compression Preliminary Results

  • Captured 2, 5 day periods of full time personal usage of Pixel 4 and replayed the logs offline
  • Compression vs Chatty:
    • 3.5x more log messages on average
    • 50% less CPU usage
    • 50% less memory usage

Log Messages in 1MB

  • The number of log messages still available in logcat after ‘Message Count’ messages have been logged to a 1MB log buffer
  • Note: ‘Simple’ is the Chatty code without log spam detection and without de-duplication.

Total Log Count

CPU Time

  • Total CPU time on ARM64 (Walleye) and 32bit x86 (Cuttlefish)
  • X axis represents different log buffer size configurations.
    • Chatty uses significantly more CPU time at 1MB (the default Pixel configuration)
    • Chatty scales poorly with increased log buffer sizes
  • Note: “simple” isn’t “compression without actually compressing”, it’s “chatty without doing the chatty elimination”, which is why “simple” is more expensive than “compression” on walleye.

CPU Time Walleye CPU Time Cuttlefish

Memory Usage

  • The memory used by ‘Message Count’ messages, on both Walleye and Cuttlefish
  • Note: Chatty does not consider the metadata (UID, PID, timestamp, etc) in its calculation of log buffer size, so a 1MB log buffer will consume more than 1MB. Note that there are 8 log buffers, 5 of which are typically filled.

Memory Usage