encoding/json: use pooled encoder in Encoder.Encode
Due to the lack of MarshalWrite in the v1 API,
it is unfortunately common to see:
json.NewEncoder(out).Encode(in)
where a single-use Encoder is constructed and thrown away.
This performed acceptably in v1 since every call to Encode
used a globally pooled encoder resource.
Prior to this change, the v1-in-v2 implementation relied
on a bytes.Buffer cached only for the lifetime of the Encoder
object itself. Thus, a single-use Encoder does not benefit.
Modify the wrapper implementation to use the internal
pooled encoder from v2 and use the intermediate buffer
to write directly to the output io.Writer.
We assume that the user-provided io.Writer never leaks the buffer,
but this assumption was already held in the v1 implementation.
We are not increasing the surface area of data corruption risk.
Performance of v1 to v1-in-v2 (before the pool fix):
name old time/op new time/op delta
NewEncoderEncode-32 30.2ms ± 4% 28.3ms ± 9% -6.19% (p=0.002 n=9+10)
name old alloc/op new alloc/op delta
NewEncoderEncode-32 7.64MB ± 0% 28.37MB ± 0% +271.23% (p=0.000 n=10+10)
name old allocs/op new allocs/op delta
NewEncoderEncode-32 200k ± 0% 100k ± 0% -49.99% (p=0.000 n=9+10)
Interestingly, v1-in-2 is slightly faster,
but the amount of allocated memory is massive.
Performance of v1 to v1-in-v2 (after the pool fix):
name old time/op new time/op delta
NewEncoderEncode-32 30.2ms ± 4% 24.0ms ± 7% -20.36% (p=0.000 n=9+10)
name old alloc/op new alloc/op delta
NewEncoderEncode-32 7.64MB ± 0% 4.09MB ± 3% -46.52% (p=0.000 n=10+10)
name old allocs/op new allocs/op delta
NewEncoderEncode-32 200k ± 0% 100k ± 0% -50.00% (p=0.000 n=9+9)
Now, the v1-in-v2 implementation is better than v1 on all metrics.
Fixes #75026
Change-Id: I50c975b1d5b8da806e46bc627966b0a39c1817eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/740660 Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>