#### Atomic Integers

What’s faster than locks? Compare-and-swap. Modern CPUs have multiple cores. As
such, all modern CPUs must have tools for performing absolutely atomic
operations to allow those multiple cores to work together. One of those
operations is the compare-and-swap or `cas`

operation.

In the abstract, a `cas`

operation takes three arguments, a variable to modify,
a given value, and a new value. The variable is set to the new value modified
only if the current value held by the variable is equal to the given value. And
this is performed as a single operation guaranteed to be safe from interference
by any other concurrent operation. Along the way, the system sets a flag marking
the success or failure of the operation. If the variable has a different value
than expected, the variable is left unchanged and the operation fails. To
complete an atomic change, you repeatedly run a calculation and end with a `cas`

operation until it succeeds. That may not sound very efficient, but it turns
out that in most cases, it is faster than locks.

Raku provides direct access to this operation through the
`cas`

function on the
`atomicint`

type and also through a
number of operators which help you perform these atomic changes.

To demonstrate one possible use of `atomicint`

, let’s first consider the ATM
problem. Two people have access to a bank account. Let’s say the account has
$1000 in it at the beginning of the day. Person One withdraws $100 from an ATM
in city center and Person Two deposits $250 at the ATM in the airport. At the
end of the day, we clearly want the new balance to be $1150. However, if both
transactions happen simultaneously, that is not guaranteed unless we take some
care to guarantee it.

Let’s start by writing our code in the naïve way:

```
my Int $balance = 1000;
start { $balance = $balance - 100 } # one
start { $balance = $balance + 250 } # two
say $balance;
```

Unfortunately, it is now possible to end up with a balance that is different form $1150. This is because if these two tasks actually run simultaneously, they perform operations that could be interleaved like this:

- Block one reads a
`$balance`

of 1000. - Block two reads a
`$balance`

of 1000. - Block one subtracts 100 from 1000 to get 900.
- Block two adds 250 to 1000 to get 1250.
- Block one sets the
`$balance`

to 900 for one. - Block two sets the
`$balance`

to 1250 for two.

The read and write operations must be executed sequentially in order for this to code to work correctly. We could use a lock or semaphore or some other traditional construct to form a critical section around modifications to the variable, but these are typically pretty dang slow.

Instead, we can use a `cas`

operation to perform the operation sequentially. So,
if we rewrite the above code using `atomicint`

we end up with this:

```
my atomicint $balance = 1000;
start { $balance ⚛️= ⚛️$balance - 100 } # one
start { $balance ⚛️= ⚛️$balance + 250 } # two
say $balance;
```

Here the result will *always* be the expected 1150. Let’s assume these two tasks
run simultaneously exactly as before, but the final assignment is a
compare-and-swap operation rather than a regular set. It would play out
like this:

- Block one reads a
`$balance`

of 1000. - Block two reads a
`$balance`

of 1000. - Block one subtracts 100 from 1000 to get 900.
- Block two adds 250 to 1000 to get 1250.
- Block one performs compare-and-swap
`$balance`

from 1000 to 900 and succeeds. - Block two performs compare-and-swap
`$balance`

from 1000 to 1250 and fails. - Block two reads a
`$balance`

of 900. - Block two adds 250 to 900 to get 1150.
- Block two performs compare-and-swap
`$balance`

from 900 to 1150 and succeeds.

The extra steps 7-9 occur because in a `cas`

operation, the resolution of
failure requires repeating the operation until it succeeds. Unless the levels of
contention for write to a single variable are extreme, this should not lead to
any task failing indefinitely.

If you don’t like emoji in your code, that’s fine. There’s a Texas function for
performing each of the operations provide by an atomic emoji operator. The
`atomicint`

provides the following operations:

`atomic-assign`

or ⚛️=`atomic-fetch`

or the ⚛️ prefix for performing atomic reads of the value`atomic-fetch-inc`

or ⚛️++ postfix`atomic-fetch-dec`

or ⚛️– postfix`atomic-fetch-add`

or ⚛️+=`atomic-fetch-sub`

or ⚛️-=`atomic-inc-fetch`

or ++⚛️ prefix`atomic-dec-fetch`

or –⚛️ prefix`cas`

Let’s quickly consider the `cas`

function itself which underlies the
implementation for the other operators. This operation allows you to implement
any operation that involves a compare-and-swap of an `atomicint`

. It two basic
forms and just to demonstrate how they work, we can re-implement our ATM problem
from above using both forms.

First, consider this program using the `cas`

function:

```
my atomicinc $balance = 1000;
sub update-balance($change-by) {
my $new-balance;
loop {
my $old-balance = $balance;
$new-balance = $balance + $change-by;
if cas($balance, $old-balance, $new-balance) == $old-balence {
last;
}
}
return $new-balance;
}
start update-balance(-100); # one
start update-balance(250); # two
say $balance;
```

This is functionally identical, though more verbose, than the operation above.
This is basically giving you direct access to the `cas`

operation itself and
lets you control the number of retries and the specific operation performed.
This function works something like this, though the entire operation is atomic:

```
multi sub cas(atomicint $target is rw, int $expected-value, int $new-value --> int) {
my int $seen = $target;
if $seen == $expected-value {
$target = $new-value;
}
return $seen;
}
```

That means it will return the value it saw in every case, but will have changed the value stored in the target when what it saw matches what you expected.

There is a second version of this method, which can help streamline your code a bit by performing the looping for you. To write the ATM problem yet again, we can also write it like this:

```
my atomicint $balance = 1000;
sub update-balance($change-by) {
cas $balance, -> $seen-value { $seen-value + $change-by }
}
start update-balance(-100); # one
start update-balance(250); # two
say $balance;
```

This greatly shortens the code required to implement this operation. It does so
by performing the given operation repeatedly in a loop until the `cas`

operation
succeeds. In this case, the `cas`

function is defined like this, but again, using
atomic operations where appropriate:

```
multi sub cas(atomicint $target is rw, &operation) {
loop {
my int $seen = $target;
my $new-value = operation($seen);
if $seen == $target { # still equal?
$target = $new-value;
}
return $new-value;
}
}
```

Be aware that the `cas`

subroutine alternative that takes a function returns the
new value that was set, while the `cas`

that takes two integers returns the
value that was seen. That is, this second and compact form treats the return
value like an assignment operation, but the way the other form works does not
allow for that treatment.

I’d really like to share with you how `cas`

can be used on any scalar value in
Raku, but this post has already gone long. I will cover that later this Advent.

Cheers.