[swift-users] Object size on heap

[Howard Lovatt]

@Kate,

I don't seem to be able to get `malloc_...` to work. EG:

class TestClass {
    let a = 0
    let b: Int? = nil
}
@_silgen_name("swift_class_getInstanceExtents") func
swift_class_getInstanceExtents(theClass: AnyClass) -> (negative: UInt,
positive: UInt)
print("swift_class_getInstanceExtents =
\(swift_class_getInstanceExtents(TestClass))")
let requiredSize = malloc_size(unsafeAddressOf(TestClass()))
print("malloc_size = \(requiredSize)")
print("malloc_good_size = \(malloc_good_size(requiredSize))")


Prints:

swift_class_getInstanceExtents = (0, 33)
malloc_size = 0
malloc_good_size = 16


The `swift_class_getInstanceExtents` seems correct to me: 16 bytes for
class overhead + 16 bytes for `a` and `b` + 1 byte because `b` is an
optional = 33 bytes.

Not sure what `malloc_...` is giving?

  -- Howard.

On 24 March 2016 at 10:39, Kate Stone <k8stone at apple.com> wrote:

> I definitely concur that tools like Instruments are the best way to
> understand the impact of decisions on memory across the board.  For
> fine-grained inquiries about memory use you can also rely on malloc family
> functions to make inquiries:
>
> let required_size = malloc_size(unsafeAddressOf(*object_reference*))
> let actual_size = malloc_good_size(required_size)
>
>
> Kate Stone k8stone at apple.com
>  Xcode Low Level Tools
>
> On Mar 23, 2016, at 3:59 PM, Howard Lovatt via swift-users <
> swift-users at swift.org> wrote:
>
> Thanks, I will give that a try
>
>   -- Howard.
>
> On 24 March 2016 at 03:17, Jens Alfke <jens at mooseyard.com> wrote:
>
>>
>> On Mar 22, 2016, at 11:04 PM, Howard Lovatt via swift-users <
>> swift-users at swift.org> wrote:
>>
>> I am writing custom collection classes and trying to assess which one is
>> better, both in terms of performance and memory usage. Won't be used in
>> 'real' code, just to guide development.
>>
>>
>> You might consider using heap profiling tools too, like (on Mac OS) the
>> Instruments app or the `heap` command-line tool. If you use these while
>> running a benchmark app using your API, it can show you how much total heap
>> space gets used.
>>
>> Actual heap usage can differ from the raw “sizeof” a data type, since
>> allocators will often round up block sizes or return a somewhat larger
>> block than necessary. Heap fragmentation can also increase memory usage
>> beyond what you’d expect, and different allocation patterns can affect
>> fragmentation.
>>
>> —Jens
>>
>
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>
>
>
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