# build parallelism?
**URL:** https://discourse.cmake.org/t/build-parallelism/6606
**Category:** Usage
**Tags:** os:linux
**Created:** [October 5, 2022, 6:20pm UTC](https://discourse.cmake.org/t/build-parallelism/6606 "2022-10-05T18:20:57Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@Mikael\_Persson](https://discourse.cmake.org/u/Mikael_Persson)
#### Post date: [October 5, 2022, 6:20pm UTC](https://discourse.cmake.org/t/build-parallelism/6606/1 "2022-10-05T18:20:57Z")
Consider the example:
```auto
CASE1)
add_library(liba ...) #nothing exotic, no generated, etc
add_library(libb ...) # only includes headers from liba, but the functions have to be there linktime
target_link_library(libb liba) #
add_executable(main ...) # only includes headers from libb
target_link_libraries(main libb)
Case2)
add_library(liba ...) #nothing exotic, no generated, etc
add_library(libb ...) # only includes headers from liba, but the functions have to be there linktime
target_include_directories(libb ${liba_include_dirs})
add_executable(main ...)
target_link_libraries(main liba libb)
```
Case1, Case2 both build executable main without issue
The Case1 build is entirely sequential,
The Case2 build parallel builds liba, libb,
There is a obvious problem with case 2, but its very hard to argue with “but case2 builds two times faster”, and its about 40 times faster in the real case,
How can I get the parallelism of case 2 while keeping the propagation of properties, etc. I’ve seen somewhere that ninja might get around this, but a sufficient portion of the coders refuse to know what that is, so it has to be a pure cmake solution, we use cmake 3.175 minimum, but I think I can bump that to 3,18 at least if needed. Ideas?
---
### Author:  [@craig.scott](https://discourse.cmake.org/u/craig.scott)
#### Post date: [October 7, 2022, 5:41am UTC](https://discourse.cmake.org/t/build-parallelism/6606/2 "2022-10-07T05:41:25Z")
If you can stretch up to CMake 3.19, the [OPTIMIZE\_DEPENDENCIES](https://cmake.org/cmake/help/latest/prop_tgt/OPTIMIZE_DEPENDENCIES.html) target property might be what you’re looking for. If your real project satisfies the criteria described in the docs for that property, then even for case 1, `liba` and `libb` should compile in parallel. The only caveat I’m aware of is [this issue](https://gitlab.kitware.com/cmake/cmake/-/issues/21517) which affects Ninja only (not dangerous, just ends up potentially rebuilding things when it doesn’t need to).
---
### Author:  [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [October 7, 2022, 5:35pm UTC](https://discourse.cmake.org/t/build-parallelism/6606/3 "2022-10-07T17:35:45Z")
This is an artifact of CMake’s guarantees. Basically, if target B uses target A, A is historically waited on to link before building anything in B because if A has custom commands to, say, generate headers, this guarantees it happens first. In [Issue 15555](https://gitlab.kitware.com/cmake/cmake/-/issues/15555), Ninja generators learned to break this dependency when it can. It’s not perfect, but there are other issues linked to there that document future progress on that front.
---
### Author:  [@Mikael\_Persson](https://discourse.cmake.org/u/Mikael_Persson)
#### Post date: [October 8, 2022, 8:03am UTC](https://discourse.cmake.org/t/build-parallelism/6606/4 "2022-10-08T08:03:58Z")
Yeah i know its an error in cmake,
But it was fixed in 3.19 as per craigs answer. There is zero reason why it would not be default, but thats just a one liner that will be in every cmake script i write till it becomes default.
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### Author:  [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)
#### Post date: [October 9, 2022, 1:23pm UTC](https://discourse.cmake.org/t/build-parallelism/6606/5 "2022-10-09T13:23:04Z")
> [@Mikael\_Persson](#):
>
> There is zero reason why it would not be default
There are probably projects out there that rely on CMake’s stronger guarantees to be correct in a parallel build. Changing the default means breaking all of those projects. There’s also no good way for _CMake_ to know that there are undeclared dependencies in the build graph to warn about such things.
---
### Author:  [@Mikael\_Persson](https://discourse.cmake.org/u/Mikael_Persson)
#### Post date: [October 10, 2022, 10:23pm UTC](https://discourse.cmake.org/t/build-parallelism/6606/6 "2022-10-10T22:23:44Z")
Hmm, fair point, but had it been the default from the start it would have been easier to let the exceptionally rare case be something you opt into imo.