# mem_dbg
[](https://crates.io/crates/mem_dbg)
[](https://crates.io/crates/mem_dbg/reverse_dependencies)
[](https://github.com/zommiommy/mem_dbg-rs)
[](https://crates.io/crates/mem_dbg)
[](https://docs.rs/mem_dbg)
Traits and associated procedural macros to display recursively the layout and
memory usage of a value.
The trait [`MemDbg`] can be used to display the recursive layout of a value,
together with the size of each part and the associated padding bytes. We provide
implementations for most basic types, a derive macro for structs and enums
whose fields implement [`MemDbg`], and support for a few other crates.
To compute the size, we provide the trait [`MemSize`] and a derive macro that
can be used to compute the size of a value in bytes as the standard library
function [`std::mem::size_of`] returns the stack size of a type in bytes, but it
does not take into consideration heap memory.
## Why `MemSize`
Other traits partially provide the functionality of [`MemSize`], but either they
require implementing manually a trait, which is prone to error, or they do not
provide the flexibility necessary for [`MemDbg`]. Most importantly, [`MemSize`]
uses the type system to avoid iterating over the content of a container (a
vector, etc.) when it is not necessary, making it possible to compute instantly
the size of values occupying hundreds of gigabytes of heap memory.
This is the result of the benchmark `bench_hash_map` contained in the `examples`
directory. It builds a hash map with a hundred million entries and then measures
its heap size:
```test
Allocated: 2281701509
get_size: 1879048240 152477833 ns
deep_size_of: 1879048240 152482000 ns
size_of: 2281701432 152261958 ns
mem_size: 2281701424 209 ns
```
The first line is the number of bytes allocated by the program as returned by
[`cap`]. Then, we display the result of [`get-size`], [`deepsize`], [`size-of`],
and our own [`MemSize`]. Note that the first two crates are just measuring the
space used by the items, and not by the data structure (i.e., they are not
taking into account the load factor and the power-of-two size constraint of the
hash map). Moreover, all other crates are about six orders of magnitude slower
than our implementation, due to the necessity to iterate over all elements.
## Padding
The trait [`MemDbg`] is useful to display the layout of a value and understand
how much memory is used by each part. In particular, it exploits the new stable
macro [`std::mem::offset_of`] to display the padding of each field in square
brackets; moreover, the flag [`DbgFlags::RUST_LAYOUT`] makes it possible to
display structures in the layout used by the Rust compiler, rather than
that given by declaration order.
These features are also available for enums using the feature `offset_of_enum`,
which however needs the nightly compiler, as it enables the unstable features
`offset_of_enum` and `offset_of_nested`.
## Features
- `offset_of_enum`: support for padding and for the `DbgFlags::RUST_LAYOUT` flag
for enums. Requires the nightly compiler as it enables the unstable features
`offset_of_enum` and `offset_of_nested`. Calling `mem_dbg` with the flag
`DbgFlags::RUST_LAYOUT` without this feature enabled will result in a panic.
- `half`: support for the [`half`] crate.
- `maligned`: support for the [`maligned`] crate.
- `mmap-rs`: support for the [`mmap-rs`] crate.
- `rand`: support for the [`rand`] crate.
## Example
```rust
# #![cfg_attr(feature = "offset_of_enum", feature(offset_of_enum, offset_of_nested))]
# fn main() -> Result<(), Box<dyn std::error::Error>> {
use mem_dbg::*;
#[derive(MemSize, MemDbg)]
struct Struct<A, B> {
a: A,
b: B,
test: isize,
}
#[derive(MemSize, MemDbg)]
struct Data<A> {
a: A,
b: Vec<i32>,
c: (u8, String),
}
#[derive(MemSize, MemDbg)]
enum TestEnum {
Unit,
Unit2(),
Unit3 {},
Unnamed(usize, u8),
Named { first: usize, second: u8 },
}
let b = Vec::with_capacity(100);
let s = Struct {
a: TestEnum::Unnamed(0, 16),
b: Data {
a: vec![0x42_u8; 700],
b,
c: (1, "foo".to_owned()),
},
test: -0xbadf00d,
};
println!("size: {}", s.mem_size(SizeFlags::default()));
println!("capacity: {}", s.mem_size(SizeFlags::CAPACITY));
println!();
s.mem_dbg(DbgFlags::empty())?;
println!();
println!("size: {}", s.mem_size(SizeFlags::default()));
println!("capacity: {}", s.mem_size(SizeFlags::CAPACITY));
println!();
s.mem_dbg(DbgFlags::default() | DbgFlags::CAPACITY | DbgFlags::HUMANIZE)?;
#[cfg(feature = "offset_of_enum")]
{
println!();
println!("size: {}", s.mem_size(SizeFlags::default()));
println!("capacity: {}", s.mem_size(SizeFlags::CAPACITY));
println!();
s.mem_dbg(DbgFlags::empty() | DbgFlags::RUST_LAYOUT)?;
}
# Ok(())
# }
```
The previous program prints:
```test
size: 807
capacity: 1207
807 B ⏺
16 B ├╴a
│ ├╴Variant: Unnamed
8 B │ ├╴0
1 B │ ╰╴1
783 B ├╴b
724 B │ ├╴a
24 B │ ├╴b
35 B │ ╰╴c
1 B │ ├╴0 [7B]
27 B │ ╰╴1
8 B ╰╴test
size: 807
capacity: 1207
1.207 kB 100.00% ⏺: readme::main::Struct<readme::main::TestEnum, readme::main::Data<alloc::vec::Vec<u8>>>
16 B 1.33% ├╴a: readme::main::TestEnum
│ ├╴Variant: Unnamed
8 B 0.66% │ ├╴0: usize
1 B 0.08% │ ╰╴1: u8
1.183 kB 98.01% ├╴b: readme::main::Data<alloc::vec::Vec<u8>>
724 B 59.98% │ ├╴a: alloc::vec::Vec<u8>
424 B 35.13% │ ├╴b: alloc::vec::Vec<i32>
35 B 2.90% │ ╰╴c: (u8, alloc::string::String)
1 B 0.08% │ ├╴0: u8 [7B]
27 B 2.24% │ ╰╴1: alloc::string::String
8 B 0.66% ╰╴test: isize
```
If run with the feature `offset_of_enum`, it prints:
```text
size: 807
capacity: 1207
807 B ⏺
16 B ├╴a
│ ├╴Variant: Unnamed
8 B │ ├╴0
1 B │ ╰╴1 [6B]
783 B ├╴b
724 B │ ├╴a
24 B │ ├╴b
35 B │ ╰╴c
1 B │ ├╴0 [7B]
27 B │ ╰╴1
8 B ╰╴test
size: 807
capacity: 1207
1.207 kB 100.00% ⏺: readme::main::Struct<readme::main::TestEnum, readme::main::Data<alloc::vec::Vec<u8>>>
16 B 1.33% ├╴a: readme::main::TestEnum
│ ├╴Variant: Unnamed
8 B 0.66% │ ├╴0: usize
1 B 0.08% │ ╰╴1: u8 [6B]
1.183 kB 98.01% ├╴b: readme::main::Data<alloc::vec::Vec<u8>>
724 B 59.98% │ ├╴a: alloc::vec::Vec<u8>
424 B 35.13% │ ├╴b: alloc::vec::Vec<i32>
35 B 2.90% │ ╰╴c: (u8, alloc::string::String)
1 B 0.08% │ ├╴0: u8 [7B]
27 B 2.24% │ ╰╴1: alloc::string::String
8 B 0.66% ╰╴test: isize
size: 807
capacity: 1207
807 B ⏺
783 B ├╴b
724 B │ ├╴a
24 B │ ├╴b
35 B │ ╰╴c
1 B │ ├╴0 [7B]
27 B │ ╰╴1
16 B ├╴a
│ ├╴Variant: Unnamed
1 B │ ├╴1 [6B]
8 B │ ╰╴0
8 B ╰╴test
```
## Caveats
- We support out-of-the-box most basic types, and tuples up to size ten. The
derive macros `MemSize`/`MemDbg` will generate implementations for structs and
enums whose fields implement the associated interface: if this is not the case
(e.g., because of the orphan rule) one can implement the traits manually.
- If you invoke the methods of this crate on a shared reference, the compiler
will automatically dereference it, and the method will be invoked on the
referenced type:
```rust
# fn main() -> Result<(), Box<dyn std::error::Error>> {
use mem_dbg::*;
let mut x: [i32; 4] = [0, 0, 0, 0];
assert_eq!(
(&x).mem_size(SizeFlags::default()),
std::mem::size_of::<[i32; 4]>()
);
assert_eq!(
(&mut x).mem_size(SizeFlags::default()),
std::mem::size_of::<&mut [i32; 4]>()
);
assert_eq!(
<&[i32; 4] as MemSize>::mem_size(&&x, SizeFlags::default()),
std::mem::size_of::<&[i32; 4]>()
);
# Ok(())
# }
```
- Computation of the size of arrays, slices, and vectors will be performed by
iterating over their elements unless the type is a copy type that does not
contain non-`'static` references and it is declared as such using the attribute
`#[copy_type]`. See [`CopyType`] for more details.
- The content of vectors and slices is not expanded recursively as the output
might be too complex; this might change in the future (e.g., via a flag)
should interesting use cases arise.
- `BTreeMap`/`BTreeSet` are not currently supported as we still have to
figure out a way to precisely measure their memory size and capacity.
[`MemDbg`]: <https://docs.rs/mem_dbg/latest/mem_dbg/trait.MemDbg.html>
[`MemSize`]: <https://docs.rs/mem_dbg/latest/mem_dbg/trait.MemSize.html>
[`std::mem::size_of`]: <https://doc.rust-lang.org/std/mem/fn.size_of.html>
[`DbgFlags::RUST_LAYOUT`]: <https://docs.rs/mem_dbg/latest/mem_dbg/struct.DbgFlags.html#associatedconstant.RUST_LAYOUT>
[`CopyType`]: <https://docs.rs/mem_dbg/latest/mem_dbg/trait.CopyType.html>
[`cap`]: <https:/crates.io/crates/cap>
[`get-size`]: <https://crates.io/crates/get_size>
[`deepsize`]: <https://crates.io/crates/deepsize>
[`size-of`]: <https://crates.io/crates/size_of>
[`maligned`]: <https://crates.io/crates/maligned>
[`mmap-rs`]: <https://crates.io/crates/mmap-rs>
[`half`]: <https://crates.io/crates/half>
[`rand`]: <https://crates.io/crates/rand>