what's the fastest way for clearing a map

[Daniel Mal]

my app made use of maps frequently, it's like

1. put some k,v into map

2. process the map

3. loop delete each key in the map, and goto step 1

what is the fastest way for clearing a map, and also i don't want to make(map) each time to give GC too much pressure.

[James Wendel]

There's no simple and easy way to clear a map that I'm aware of. It is normally just easier to create a new one. Alternatively, are you able to solve your problem using arrays/slices?

[Sameer Ajmani]

If your goal is to create an empty map that can store the same number of keys as the map used in the previous iteration, pass the desired size for the new map as the second parameter to make.  That will reduce or eliminate the number of times the map is resized each round.

But if you have relatively few key collisions each round, it might be more efficient to append your items to a slice then sort them at the end to identify duplicates.  Then just reslice down to zero at the start of each round to reuse the underlying array.

Profile your code and see.

On Aug 3, 2014 9:38 AM, "James Wendel" <jmwe...@gmail.com> wrote:

There's no simple and easy way to clear a map that I'm aware of. It is normally just easier to create a new one. Alternatively, are you able to solve your problem using arrays/slices?

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[k...@golang.org]

for k := range m { delete(m, k) }

should work fine.  It will probably be faster to create a new (correctly sized, if you know it) map, but reusing can put less pressure on the garbage collector.

[Rob Pike]

Does it always put significantly less pressure on the collector to run
the loop? The amount certainly depends on the types of the key and
value, since a large map of complex types can contain a lot of
pointers and such. The overhead of the buckets could be anything from
dominant to minor as I understand things.

For a simple map, like map[string]int, it's likely that the bucket
overhead is relatively large.

-rob

[dja...@gmail.com]

if i have server that process N (big value) maps per second,  to reuse or make new map ?

[Sameer Ajmani]

Perhaps use a sync.Pool to recycle maps in your server.

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[Dave Cheney]

make() a new one is faster than emptying the old one. Both result in the same amount of garbage, but the former does less paperwork beforehand.

[Brad Fitzpatrick]

On Mon, Aug 4, 2014 at 5:29 PM, Dave Cheney <da...@cheney.net> wrote:

make() a new one is faster than emptying the old one. Both result in the same amount of garbage, but the former does less paperwork beforehand.

Why does emptying the old one make any garbage?

 

[Dave Cheney]

I had assumed that the OP was talking about a map with indirect keys/values, in that case I believe those keys/values will become garbage after deletion. At least in 1.4 when there is no more free method inside the runtime.