GHC's performance

Johan Tibell johan.tibell at
Fri Apr 11 10:55:13 UTC 2014

On Fri, Apr 11, 2014 at 10:58 AM, Simon Marlow <marlowsd at> wrote:

> Let me second this.  In particular, I think we regress on GHC performance
> regularly, because the perf tests just aren't a good way to prevent
> regressions.  When we have a +/- 10% window, someone can commit a 9%
> regression without triggering a perf failure, but the next patch to come
> along with a 1% regression will be unfairly blamed.

Agreed. I think graphing results helps here. It's often easier to visualize
identify which commit is the real culprit.

Aside: I think moving completely to subrepos will generally help us track
down regressions, both performance and correctness, faster. Being able to
`git bisect` your way to the cause saves a lot of time.

Furthermore, by the time we get to the perf tests we're nearly done and
> just want to push and go home, not go back and profile GHC.  Yet the perf
> tests have an important purpose: the idea is to catch the problem when we
> have the crucial piece of information: the patch that caused the
> regression.  Someone can try to optimise GHC later, but they have to start
> from scratch without the information about what caused the regressions.
> Having an automated system to track GHC performance would help a lot with
> this, I think.

Agree a 100%. Automation is what's needed here.
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