Tag: profiling

Latency Numbers Every Programmer Should Know

latency

I’m saving this here for current and future generations of programmers:

Latency Comparison Numbers
--------------------------
L1 cache reference                            0.5 ns
Branch mispredict                             5   ns
L2 cache reference                            7   ns             14x L1 cache
Mutex lock/unlock                            25   ns
Main memory reference                       100   ns             20x L2 cache, 200x L1 cache
Compress 1K bytes with Zippy              3,000   ns
Send 1K bytes over 1 Gbps network        10,000   ns    0.01 ms
Read 4K randomly from SSD*              150,000   ns    0.15 ms
Read 1 MB sequentially from memory      250,000   ns    0.25 ms
Round trip within same datacenter       500,000   ns    0.5  ms
Read 1 MB sequentially from SSD*      1,000,000   ns    1    ms  4X memory
Disk seek                            10,000,000   ns   10    ms  20x datacenter roundtrip
Read 1 MB sequentially from disk     20,000,000   ns   20    ms  80x memory, 20X SSD
Send packet CA->Netherlands->CA     150,000,000   ns  150    ms

Notes
-----
1 ns = 10-9 seconds
1 ms = 10-3 seconds
* Assuming ~1GB/sec SSD

Credit
------
By Jeff Dean:               http://research.google.com/people/jeff/
Originally by Peter Norvig: http://norvig.com/21-days.html#answers

Contributions
-------------
Some updates from:                      https://gist.github.com/2843375
Great 'humanized' comparison version:   https://gist.github.com/2843375
Visual comparison chart:                http://i.imgur.com/k0t1e.png
Nice animated presentation of the data: http://prezi.com/pdkvgys-r0y6/latency-numbers-for-programmers-web-development/

This is a copy-paste of this gist, referenced from this blog post. Read and share both, for the better world.

Profiling perl

After I’ve complained on #perl channel that dprofpp (perl profiler) dies on me, I received a number of suggestions on how to profile the code without using dprofpp. Ideas ranged from elemetary insertion of ‘print “Started”, time,”\n”‘ to all sorts of dynamic subroutine overloading. Here is one elegant solution that was suggested:

sub foo { print "foo (@_)\n" }
sub bar { print "bar (@_)\n" }

for (qw(foo bar)) {
  my $name = "main::" . $_;
  my $old = *$name{CODE};
  *$name = sub {
    print scalar(localtime), " Started $name\n";
    &$old;
    print scalar(localtime), " Finished $name\n";
  };
}

foo(qw(one two));
bar(qw(three four));

Oh, and someone also mentioned that dprofpp is known to be buggy. Here is a quote from the “Profiling Perl” article at perl.com:

The other problem you might face is that Devel::DProf, being somewhat venerable, occasionally has problems keeping up on certain recent Perls, (particularly the 5.6.x series) and may end up segfaulting all over the place.