x/net/http2: reduce allocations of (*clientConnReadLoop).handleReponse by 10%

[rs]

What version of Go are you using (go version)?

1.14

What did you do?

In our deployment, (*clientConnReadLoop).handleReponse is the 4th method creating the most allocations.

By applying the same slice trick as in textproto.(*Reader).ReadMIMEHeader, we can save about 10% of the allocations:

name                                   old time/op    new time/op    delta
ClientResponseHeaders/___0_Headers-16    82.3µs ± 7%    76.4µs ± 4%   -7.18%  (p=0.000 n=10+10)
ClientResponseHeaders/__10_Headers-16     101µs ± 2%      99µs ± 3%   -2.00%  (p=0.016 n=8+10)
ClientResponseHeaders/_100_Headers-16     213µs ± 2%     202µs ± 4%   -4.96%  (p=0.000 n=9+9)
ClientResponseHeaders/1000_Headers-16    2.28ms ± 1%    2.15ms ± 2%   -5.58%  (p=0.000 n=8+10)

name                                   old alloc/op   new alloc/op   delta
ClientResponseHeaders/___0_Headers-16    4.60kB ± 0%    4.60kB ± 0%     ~     (p=0.201 n=10+10)
ClientResponseHeaders/__10_Headers-16    9.01kB ± 0%    8.66kB ± 0%   -3.96%  (p=0.000 n=10+10)
ClientResponseHeaders/_100_Headers-16    54.4kB ± 0%    48.4kB ± 0%  -11.01%  (p=0.000 n=10+10)
ClientResponseHeaders/1000_Headers-16     702kB ± 0%     595kB ± 0%  -15.28%  (p=0.000 n=10+9)

name                                   old allocs/op  new allocs/op  delta
ClientResponseHeaders/___0_Headers-16      57.0 ± 0%      56.0 ± 0%   -1.75%  (p=0.000 n=10+10)
ClientResponseHeaders/__10_Headers-16       135 ± 0%       123 ± 0%   -8.89%  (p=0.000 n=10+10)
ClientResponseHeaders/_100_Headers-16       786 ± 0%       679 ± 0%  -13.61%  (p=0.000 n=10+10)
ClientResponseHeaders/1000_Headers-16     8.14k ± 0%     7.11k ± 0%  -12.65%  (p=0.000 n=10+10)

Here is the patch:

diff --git a/http2/transport.go b/http2/transport.go
index 81778be..e4fb025 100644
--- a/http2/transport.go
+++ b/http2/transport.go
@@ -1892,7 +1892,9 @@ func (rl *clientConnReadLoop) handleResponse(cs *clientStream, f *MetaHeadersFra
                return nil, errors.New("malformed response from server: malformed non-numeric status pseudo header")
        }

-       header := make(http.Header)
+       regularFields := f.RegularFields()
+       strs := make([]string, len(regularFields))
+       header := make(http.Header, len(regularFields))
        res := &http.Response{
                Proto:      "HTTP/2.0",
                ProtoMajor: 2,
@@ -1900,7 +1902,7 @@ func (rl *clientConnReadLoop) handleResponse(cs *clientStream, f *MetaHeadersFra
                StatusCode: statusCode,
                Status:     status + " " + http.StatusText(statusCode),
        }
-       for _, hf := range f.RegularFields() {
+       for _, hf := range regularFields {
                key := http.CanonicalHeaderKey(hf.Name)
                if key == "Trailer" {
                        t := res.Trailer
@@ -1912,7 +1914,18 @@ func (rl *clientConnReadLoop) handleResponse(cs *clientStream, f *MetaHeadersFra
                                t[http.CanonicalHeaderKey(v)] = nil
                        })
                } else {
-                       header[key] = append(header[key], hf.Value)
+                       vv := header[key]
+                       if vv == nil && len(strs) > 0 {
+                               // More than likely this will be a single-element key.
+                               // Most headers aren't multi-valued.
+                               // Set the capacity on strs[0] to 1, so any future append
+                               // won't extend the slice into the other strings.
+                               vv, strs = strs[:1:1], strs[1:]
+                               vv[0] = hf.Value
+                               header[key] = vv
+                       } else {
+                               header[key] = append(vv, hf.Value)
+                       }
                }
        }

Please tell me if it's worth a CL.

[odeke-em]

Hello @rs, great catching you here!

Thank you and Sounds good to me, please send a CL.

Perhaps let’s rename strs to valuesSlab, because that’s the parent slice that we’ll be using to create individual value slices each of capacity 1.

[rs]

Thanks. For the strs naming, I used the same as in textproto.(*Reader).ReadMIMEHeader. I thought it would be more consistent.

[gopherbot]

Change https://golang.org/cl/223783 mentions this issue: http2: reduce allocations of (*clientConnReadLoop).handleReponse

[gopherbot]

Change https://golang.org/cl/224217 mentions this issue: net/http: update bundled x/net/http2