fastcounters

Fast, persistent, atomically incrementing, cross-process counters.

Copyright (C) 2011, 2015 Asher Blum; http://wildsparx.com/fastcounters/

• Can be used by multiple related or unrelated processes.
• Can be used by multiple threads. No need to lock.
• Persistent, but lazily - if you pull the power cord, data is lost.
• Command line tool for creating and inspecting the counter file.
• Counter file contains multiple named counters.

Performance

Approx 385 million increments per second. Measured with:

• 4 processes
• 10M increments each
• 4 core i7 CPU
• 2.67GHz

Platform

Linux, Intel, GCC.

How It Works

Fastcounters uses a memory mapped file and increments the counters with the atomic xaddl instruction; the code is taken from the linux kernel.

License

GPL, since it uses code from the linux kernel.

Creating the counter file

Create a file with four anonymous counters like this:

fastcounters -n4 counters.dat

(They will be named C0 thru C3)

Or create a file with three named counters like this:

fastcounters -n cats,dogs,snakes counters.dat

Incrementing Counters

In a C program, you can increment a counter like this:

int res;
if(fcntrs_incr(&fc, 0, &res)) {
    // handle error
}

Note:

• fc is a fastcounters handle already opened.
• This increments the first (position 0) counter in the file.
• res now contains the new counter value.

Ways of Addressing Counters

There are three ways of addressing counters:

Literal Int

Shown in the example above.

Defined Constant

Given a counters file, you can generate a C fragment with:

fastcounters -poh counters.dat

This will produce output like:

#define COUNTER_cats      0
#define COUNTER_dogs      1
#define COUNTER_snakes    2

You can use these defined constants to access the counters.

Dynamic Name Lookup

You can dynamically resolve counter names to IDs:

int cnt_id;
if(fcntrs_resolve_name(fc, "dogs", &cnt_id)) {
    // handle error
}

Since this lookup is signficantly slower than incrementing a counter, the result should be cached. One way to do this is with static variables:

static int cnt_id = -1;
if(cnt_id == -1) { // first time
    if(fcntrs_resolve_name(fc, "dogs", &cnt_id)) {
...