# Crystal parallelism and Kemal **URL:** \ **Category:** Help & Support\ **Created:** [March 3, 2020, 12:14am UTC](https://forum.crystal-lang.org/t/crystal-parallelism-and-kemal/1790 "2020-03-03T00:14:06Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![CaDs](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/cads/32/66_2.png) [@CaDs](https://forum.crystal-lang.org/u/CaDs)\ **Post date:** [March 3, 2020, 12:14am UTC](https://forum.crystal-lang.org/t/crystal-parallelism-and-kemal/1790/1 "2020-03-03T00:14:06Z")
I was trying to find a good resource on how to have Kemal benefiting from Crystal’s parallelism support. I did some googling but I couldn’t find a resource on this. Do you know about any good material that covers this? Thanks ---
**Author:** ![Blacksmoke16](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/blacksmoke16/32/1241_2.png) [@Blacksmoke16](https://forum.crystal-lang.org/u/Blacksmoke16)\ **Post date:** [March 3, 2020, 12:42am UTC](https://forum.crystal-lang.org/t/crystal-parallelism-and-kemal/1790/2 "2020-03-03T00:42:38Z")
Kemal uses the standard libraries’ `HTTP::Server` module under the hood. All you should have to do is essentially just build your binary with `-D preview_mt` and that should be it. The number of worker threads defaults to `4` but an be controlled by the `CRYSTAL_WORKERS` env var. Just note that multithreading is still considered experimental, and some portions of Kemal may not be thread safe. ---
**Author:** ![CaDs](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/cads/32/66_2.png) [@CaDs](https://forum.crystal-lang.org/u/CaDs)\ **Post date:** [March 3, 2020, 1:39am UTC](https://forum.crystal-lang.org/t/crystal-parallelism-and-kemal/1790/3 "2020-03-03T01:39:44Z")
Thanks a lot. I will give it a try. I want to update the version running on [https://www.techempower.com/benchmarks/](https://www.techempower.com/benchmarks/) ---
**Author:** ![vlazar](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/vlazar/32/31_2.png) [@vlazar](https://forum.crystal-lang.org/u/vlazar)\ **Post date:** [March 3, 2020, 4:36am UTC](https://forum.crystal-lang.org/t/crystal-parallelism-and-kemal/1790/4 "2020-03-03T04:36:10Z")
The Techempower’s benchmarks already run in multiple processes via `reuse_port: true`. It’s highly unlikely the results would improve. If you do update benchmarks, please update all Crystal frameworks. It’s good to see apples to apples comparison. ---
**Author:** ![jgaskins](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/jgaskins/32/2449_2.png) [@jgaskins](https://forum.crystal-lang.org/u/jgaskins)\ **Post date:** [March 3, 2020, 3:10pm UTC](https://forum.crystal-lang.org/t/crystal-parallelism-and-kemal/1790/5 "2020-03-03T15:10:02Z")
> [@vlazar](#): > > It’s highly unlikely the results would improve. This is very true. In fact, not only would they not improve, but they may very well end up worse. Crystal is much more efficient across fibers in single-threaded mode than it is when multithreaded. It would use more CPU time to do the same amount of work with `preview_mt` enabled. Depending on your production infrastructure, either one of these could be more realistic. On Heroku, it’s easier to run a single multithreaded process to use the full computing power of the box you’re running on. On Kubernetes, it’s easy enough to assign multiple processes to a single box that you don’t have to use either multithreading or the `fork`+`reuse_port` trick. All that to say, I think it’s a good idea to have _both_ benchmarks — multithreading and multi-process. ---
**Author:** ![rogerdpack](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/rogerdpack/32/117_2.png) [@rogerdpack](https://forum.crystal-lang.org/u/rogerdpack)\ **Post date:** [March 3, 2020, 3:35pm UTC](https://forum.crystal-lang.org/t/crystal-parallelism-and-kemal/1790/6 "2020-03-03T15:35:36Z")
In my benchmarks once you get past like 4 simultaneous workers then it starts to bottleneck on the GC, FWIW… > [@Multithreaded Crystal initial thoughts](https://forum.crystal-lang.org/t/multithreaded-crystal-initial-thoughts/1089/32): > > OK here’s some patches I found to speed it up a little but overall…couldn’t see much, except that past like 4-6 threads it seems to get all choked up on global locks allocs and realloc’s…dang… (you can see by running gdb against it: gdb -ex “set pagination 0” -ex “thread apply all bt” -batch -p or using some other profiler.) At the same time it did get to like 10% idle cpu on a box with 8 cores I was using for it. Maybe that’s good? Guess there’s multi process if you need to use 100% … ---
**Author:** ![straight-shoota](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/straight-shoota/32/36_2.png) [@straight-shoota](https://forum.crystal-lang.org/u/straight-shoota)\ **Post date:** [March 3, 2020, 8:25pm UTC](https://forum.crystal-lang.org/t/crystal-parallelism-and-kemal/1790/7 "2020-03-03T20:25:53Z")
> [@Blacksmoke16](#): > > Just note that multithreading is still considered experimental, and some portions of Kemal may not be thread safe. Don’t even need to look at kemal. Stdlib’s `HTTP:::Server` is already **not threadsafe**. Multi-threading is really just a technology preview and not recommended for any productive use. It probably works fine most of the time but only very few parts of the standard library (mostly the concurrency primitives) have been revisited to make sure they’re thread-safe (even that might not be finished). As @jgaskins explained, multithreading is unlikely to outperform multi-process deployment. It it did show better results, there would be a flaw somewhere. I agree it’s still worth showing both options, but given the current preview state, I’d advise against posting any benchmarks in such a comparison. Multithreading support is a work in progress and it actually still requires quite some effort to finish. Until there’s a somewhat stable release for multithreading support in view, there’s no point in comparing performance. It would only suggest multithreading was ready to use, and probably paint an incorrect picture of its performance characteristics. ---
**Author:** ![CaDs](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/cads/32/66_2.png) [@CaDs](https://forum.crystal-lang.org/u/CaDs)\ **Post date:** [March 3, 2020, 11:53pm UTC](https://forum.crystal-lang.org/t/crystal-parallelism-and-kemal/1790/8 "2020-03-03T23:53:17Z")
aaah, I see your point. I will wait then until this is no longer a preview flag. There is no point on adding noise in those benchmarks. Thanks!