Re: Question about sparse arrays/atomic operations in systemtap

[Josh Stone <jistone at redhat dot com>]

On 06/03/2014 08:30 AM, Josh Stone wrote:
> On 06/03/2014 06:34 AM, Alan Conway wrote:
>> Another way to solve it (which might have more general use) would be to
>> have a new array type: a 'sparse array' which automatically deletes
>> values when they are set to the array default (0, "", empty statistic
>> etc) Such an array type would be very useful for arrays that are
>> potentially huge (e.g. an entry for every mutex in a process) but are
>> mostly 0.
>>
>> Is there already a solution to this problem in systemtap, or would the
>> sparse array be a useful addition?
> 
> This might still be a useful feature for brevity and performance.  I'll
> have to check, but ISTR the map runtime even has a flag to do this
> already, so we'd just have to express that desire somehow.

Just a quick followup, I think there might be a misconception behind
calling this a "sparse array" -- they're all actually maps, even when
your indexes are integers, so there's no waste for gaps in indexes.

But auto-deleting would not help if you want to still represent entries
as present-but-zero.  Presence will satisfy the "in" operator, but once
deleted that returns false.  Remembering zeroed keys so that they are
still "in" means we have to keep those keys stored somewhere.

So delete-on-zero is possible, but that wouldn't preserve "in".