Improve Performance of HTTP Request and Response inits by nerdsupremacist · Pull Request #145 · apple/swift-http-types

nerdsupremacist · 2026-08-25T12:49:01Z

Motivation

Currently constructing a request or response from parsed http fields requires multiple unnecessary allocations.
Mainly due to:

Allocations from resizing the array as we keep appending more values to it
When running validation checks, we temporarily use an array of values to represent the values for a given field name. When those fields are set this requires an extra array allocation just to run the check

Modifications

Reserve needed capacity during initial of parsed fields to prevent multiple subsequent allocations
Use lazy sequence methods to avoid allocating a brand new array when validating if all the values for a field are the same

Result

Reduction in allocations and instructions in benchmarks

Before

HTTPRequest.init(parsed)
╒═══════════════════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╕
│ Metric                │        p0 │       p25 │       p50 │       p75 │       p90 │       p99 │      p100 │   Samples │
╞═══════════════════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╡
│ Instructions (K) *    │        14 │        14 │        14 │        14 │        14 │        14 │        51 │     10000 │
├───────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Malloc (total) *      │         5 │         5 │         5 │         5 │         5 │         5 │         5 │     10000 │
╘═══════════════════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╛

HTTPResponse.init(parsed)
╒═══════════════════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╕
│ Metric                │        p0 │       p25 │       p50 │       p75 │       p90 │       p99 │      p100 │   Samples │
╞═══════════════════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╡
│ Instructions (K) *    │        18 │        18 │        18 │        18 │        18 │        19 │        41 │     10000 │
├───────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Malloc (total) *      │         7 │         7 │         7 │         7 │         7 │         7 │         7 │     10000 │
╘═══════════════════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╛

After

HTTPRequest.init(parsed)
╒═══════════════════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╕
│ Metric                │        p0 │       p25 │       p50 │       p75 │       p90 │       p99 │      p100 │   Samples │
╞═══════════════════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╡
│ Instructions (K) *    │        10 │        10 │        10 │        10 │        10 │        11 │        28 │     10000 │
├───────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Malloc (total) *      │         2 │         2 │         2 │         2 │         2 │         2 │         2 │     10000 │
╘═══════════════════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╛

HTTPResponse.init(parsed)
╒═══════════════════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╤═══════════╕
│ Metric                │        p0 │       p25 │       p50 │       p75 │       p90 │       p99 │      p100 │   Samples │
╞═══════════════════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╪═══════════╡
│ Instructions (K) *    │        12 │        12 │        12 │        12 │        12 │        12 │        36 │     10000 │
├───────────────────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┼───────────┤
│ Malloc (total) *      │         2 │         2 │         2 │         2 │         2 │         2 │         2 │     10000 │
╘═══════════════════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╧═══════════╛

…ields

tmp

fabianfett · 2026-08-25T13:53:11Z

+    func fields(for name: HTTPField.Name) -> some Sequence<HTTPField> {
        HTTPFieldSequence(fields: self, name: name)


this should still return the HTTPFieldSequence. HTTPFieldSequence should become internal though.

fabianfett · 2026-08-25T13:54:13Z

+    init(reservingCapacity capacity: Int) {
+        fields.reserveCapacity(capacity)
+    }


let's remove this and instead just call reserveCapacity on the HTTPFields.

fabianfett · 2026-08-25T13:54:39Z

+    }
+
+    init(parsed: [HTTPFields.Element]) throws {
+        let nonPseudoCount = parsed.count { !$0.name.isPseudo }


This is O(n). Let's just over reserve.

Can do. FYI I tested it and it didn't yield any difference in the benchmarks but that could also just be the benchmarks

fabianfett · 2026-08-25T13:55:16Z

+
+    init(parsed: [HTTPFields.Element]) throws {
+        let nonPseudoCount = parsed.count { !$0.name.isPseudo }
+        self.fields = HTTPFields(reservingCapacity: nonPseudoCount)


Suggested change

self.fields = HTTPFields(reservingCapacity: nonPseudoCount)

self.fields = HTTPFields()

self.fields.reserveCapacity(parsed.count)

fabianfett · 2026-08-25T13:57:11Z

+        guard self.fields.fields(for: .contentLength).allValuesSame else {
            throw ParsingError.multipleContentLength
        }
-        guard self.fields[values: .contentDisposition].allElementsSame else {
+        guard self.fields.fields(for: .contentDisposition).allValuesSame else {
            throw ParsingError.multipleContentDisposition
        }
-        guard self.fields[values: .location].allElementsSame else {
+        guard self.fields.fields(for: .location).allValuesSame else {
            throw ParsingError.multipleLocation


Urgh we walk the full field list three times here. can we just do it once?

Good callout. I have added the validation to happen while we're appending the values rather than after the fact. Given there was a lot of duplicate code I wrapped the validation in a property wrapper (might be overkill let me know and I can remove it)

fabianfett · 2026-08-25T13:58:17Z

+        for field in parsed {
+            try self.add(field: field)
+        }


We should do the pseudo headers first. for all remaining headers, we likely can use: append(contentsOf: parsed[pseudoHeaders...])

nerdsupremacist added 3 commits August 21, 2026 14:51

Add benchmarks initializing HTTP Requests and Responses from parsed f…

b49cf04

…ields

Reserve capacity for non pseudo HTTP Fields

f35c5b8

tmp

Avoid array allocations during field validation

298a019

nerdsupremacist changed the title ~~Mq http parsed fields perf~~ Improve Performance of HTTP Request and Response inits Aug 25, 2026

fabianfett reviewed Aug 25, 2026

View reviewed changes

nerdsupremacist added 2 commits August 25, 2026 15:51

Using reserve capacity method instead of custom init

c4bd83b

Validating header fields while they are being added

eb95664

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Improve Performance of HTTP Request and Response inits - #145

Improve Performance of HTTP Request and Response inits#145
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nerdsupremacist:mq-http-parsed-fields-perf

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		func fields(for name: HTTPField.Name) -> some Sequence<HTTPField> {
		HTTPFieldSequence(fields: self, name: name)

	self.fields = HTTPFields(reservingCapacity: nonPseudoCount)
	self.fields = HTTPFields()
	self.fields.reserveCapacity(parsed.count)

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nerdsupremacist commented Aug 25, 2026

Motivation

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Result

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nerdsupremacist Aug 25, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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nerdsupremacist Aug 25, 2026 •

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