Dash Profiler 200910

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Dash Profiler 200910

  1. 1. DashProfiler Lightweight Code Instrumentation Tim.Bunce@pobox.com - October 2009
  2. 2. A Problem A web application ~100K lines of code Using many external services If response time goes up... what’s causing it? Continuous monitoring in production Must have very low CPU and I/O cost Minimal code changes
  3. 3. A Typical Approach package MyNetIO; sub send_request { my ($hostname, $request) = @_ ...send to $hostname... } How much time was spent sending the request?
  4. 4. A Typical Approach package MyNetIO; use Time::Hires qw(time); sub send_request { my ($hostname, $request) = @_ my $start = time(); ...send to $hostname... $durations->{MyNetIO}{$hostname} = time() - $start; } • Doesn’t record count so can’t produce averages. • Two lines of code. Worse if multiple return statements. • Doesn’t record time if function exits via an exception. Still need to write code to flush
  5. 5. A Solution: DashProfiler Simple Flexible Lightweight
  6. 6. DashProfiler • Can group samples into granular time units • Can measure exclusive time in a period • Can flush to disk at intervals • Just needs one line of code per sample
  7. 7. DashProfiler Internals Built on DBI::Profile, part of the DBI Aggregates measurements into a data tree Two-level tree by default: $root->{ $key1 }->{ $key2 }->[ ...leaf node... ] $root->{ ‘MyNetIO’ }->{ $hostname }->[ ...leaf node... ]
  8. 8. DashProfiler Data Each leaf node in the tree is a reference to an array: $root->{ $key1 }->{ $key2 } = [ 106, # 0: count of samples at this node 0.0312958955764771, # 1: total duration 0.000490069389343262, # 2: first duration 0.000176072120666504, # 3: shortest duration 0.00140702724456787, # 4: longest duration 1023115819.83019, # 5: time of first sample 1023115819.86576, # 6: time of last sample ] First sample populates all Later samples always update 0, 1, and 6 and may update 3 or 4
  9. 9. DashProfiler By-Time Optional extra time level in the data tree $time = int(time() / $granularity) * $granularity; $root->{ $time }->{ ‘MyNetIO’ }->{ $hostname }->[ ... ] So a new sub-tree is grown each granularity seconds
  10. 10. DashProfiler Config use DashProfiler; DashProfiler->add_profile( foo => { } ); DashProfiler->add_profile( foo => { granularity => 10, flush_interval => 600, flush_hook => sub { ... }, sample_class => ‘DashProfiler::Sample’, dbi_profile_class => ‘DBI::Profile’, period_exclusive => ..., period_summary => ..., ... }); Create named profiles Lots of features
  11. 11. Without DashProfiler package MyNetIO; use Time::Hires qw(time); sub send_request { my ($hostname, $request) = @_ my $start = time(); ...send to $hostname... $durations->{MyNetIO}{$hostname} = time() - $start; }
  12. 12. Without DashProfiler package MyNetIO; use DashProfiler::Import foo_profiler => [ ‘MyNetIO’ ]; sub send_request { my ($hostname, $request) = @_ my $sample = foo_profiler( $hostname ); ...send to $hostname... } - imports a pre-curried profiler code ref DashProfiler::Import - Profilers return bless object containing timestamp - Object destruction triggers accumulation of sample
  13. 13. With DashProfiler Name of profile created with add_profile() Value to use for ‘key1’ package MyNetIO; use DashProfiler::Import foo_profiler => [ ‘MyNetIO’ ]; sub send_request { Value to use for ‘key2’ my ($hostname, $request) = @_ my $sample = foo_profiler( $hostname ); ...send to $hostname... } Duration is measured when $sample goes out of scope
  14. 14. With DashProfiler package MyNetIO; use DashProfiler::Import foo_profiler => [ ‘MyNetIO’ ]; sub send_request { my ($hostname, $request) = @_ my $sample = foo_profiler( $hostname ) if foo_profiler_enabled(); ...send to $hostname... Automatically imported compile-time constant } reduces cost to zero if profile is disabled
  15. 15. DashProfiler Flush Data is written to STDERR on exit, by default Regular flushing is enabled by specifying a flush_interval The dbi_profile_class handles the flush. Choices include: DBI::Profile DBI::ProfileData DBI::ProfileData::Apache DashProfiler->add_profile( foo => { ..., flush_interval => 600, dbi_profile_class => ‘DBI::ProfileData’, flush_hook => sub { ... }, ... });
  16. 16. DashProfiler Periods • Group samples into periods - e.g. http request to response - start_sample_period() and end_sample_period() - counted, to enable averages and totals per period - can output period counts instead of sample counts • Measure ‘exclusive’ time - time from period start to end that’s not been accounted for by other samples - enabled via period_exclusive option
  17. 17. Example Data Average response times over 24 hours DashProfiler doesn’t generate graphs itself, but the data can be used to create graphs like these
  18. 18. Example Data Worst case response times over 24 hours
  19. 19. DashProfiler Perspectives • Each DashProfiler can have multiple DBI Profile objects attached • Samples accumulate in all attached profiles • Each profile can have a different Path • giving different ‘perspectives’ or level of detail - key1 + key2 - key1 + country + browser type - key2 + browser type - ... etc.
  20. 20. DashProfiler Per-Period • Optional extra ‘per-period’ DBI profile • Enabled via period_summary option • Automatically attached and reset by start_sample_period() • Gives current totals for this period • Great for ‘debug footers’ on web page showing how much time was spent generating this page
  21. 21. DashProfiler Cost Time cost of taking a sample: 0.000021s - Time to create sample object, destroy it, accumulate the counts - In hot code can be 0.000015s - (Timings made on a 2GHz MacBook Pro Intel Core Duo) - (Could be made much faster by porting sampler class to C)
  22. 22. Questions? Tim.Bunce@pobox.com @timbunce on twitter http://blog.timbunce.org

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