[swift-server-dev] Next HTTP API meeting

Chris Bailey BAILEYC at uk.ibm.com
Mon Mar 27 09:37:32 CDT 2017

Nice work!

Taking a quick look at the project and screenshot, am I right in saying 
that there is no concurrency in the test? ARC generally has a bigger 
impact in concurrent use cases because of the need to keep memory 
consistency across processors for the atomic increment/decrement.

How hard would it be to add a dispatch queue in?


From:   Tanner Nelson via swift-server-dev <swift-server-dev at swift.org>
To:     Michael Chiu <hatsuneyuji at icloud.com>
Cc:     swift-server-dev <swift-server-dev at swift.org>
Date:   27/03/2017 14:38
Subject:        Re: [swift-server-dev] Next HTTP API meeting
Sent by:        swift-server-dev-bounces at swift.org

Re: performance,

I did a quick test of inout struct vs. class performance. The code can be 
found here: https://github.com/tanner0101/request-types

I found only a marginal increase in performance (~5%) in favor of inout 
value types. https://github.com/tanner0101/request-types/issues/1

Additionally, non-inout value types were a lot slower. This is obvious to 
the seasoned Swift dev considering each middleware in the test modifies 
and thus must copy the request. But this is the exact type of performance 
issue you can expect developers to create when interacting with 
"non-obvious value types". HTTP request/response being non-obvious value 
types compared to something like an integer or a float. (I'd argue the 
majority of web developers would expect request/response to be a reference 
type and thus easily forget or not know to use `inout`)

Please feel free to submit any prs/issues/comments about ways I could 
improve this test to make it more accurate. 

tl;dr: value types don't seem much faster than reference types (especially 
considering dangers of misuse) in a simulated web framework scenario

inb4: people saying that the request/response models in my test are 
incomplete/not fully implemented/bad. this is _not_ a proposed api for 

tanner at vapor.codes

On Mar 27, 2017, at 1:55 PM, Michael Chiu via swift-server-dev <
swift-server-dev at swift.org> wrote:

On Mar 27, 2017, at 5:13 AM, Logan Wright via swift-server-dev <
swift-server-dev at swift.org> wrote:
If people feel extremely strong that there needs to be a concrete type, 
then I'd like to push for reference type as much as possible. As far as 
reference vs value type, I haven't really heard an argument for value 
types beyond what feels like a reaction to value types being the hip new 
hotness. While yes, they're great in Swift, and there's tons of places 
that should absolutely be modeled with value semantics, a request/response 
interaction represents a single request and should definitely be a 
reference based interaction.

I disagree with this one. First of all I think most of the framework pass 
the request and response as inout argument, if that is the case there 
shouldn’t be much copy overhead in the run loop. Second the problem of 
reference type is that everywhere the request and response exists could 
possibly mutate the res/req, and it affect globally. It is true that in 
normal use there shouldn’t be two place simultaneously operate on the same 
request but that could happen. (Therefore protocol is the best isn’t it)

In practice, we went through several model iterations and the value type 
created very high levels of bugs and confusion for end users. The three 
biggest problems we had were as follows:

- Greatly increased bug levels and confusion related to unexpected 
- Unnecessary code requirements added to every single passive access (ie: 
middleware) increasing code bloat unnecessarily
- Extreme performance loss due to massive copy overhead

Each of these problems evaporated pretty instantaneously when moving to 
reference types; it made it significantly easier to reason about for end 
Just for curiosity, I’m very interested in the unexpected mutation of 
value semantic, I have always had an impression of value semantic are more 
free from unexpected mutation. 

Would like to remind again for those that skipped above reading that our 
goal is not to build a web framework here, but rather to build small tools 
that make building frameworks slightly easier for library maintainers and 
That’s so true lol.


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