lib/bytesutil: smooth buffer growth rate (#6761)

Before, buffer growth was always x2 of its size, which could lead to
excessive memory usage when processing big amount of data.
For example, scraping a target with hundreds of MBs in response could
result into hih memory spikes in vmagent because buffer has to double
its size to fit the response. See
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6759

The change smoothes out the growth rate, trading higher allocation rate
for lower mem usage at certain conditions.

---------

Signed-off-by: hagen1778 <roman@victoriametrics.com>
(cherry picked from commit f28f496a9d)

docs/CHANGELOG.md

@@ -32,6 +32,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * FEATURE: add `/influx/health` health-check handler for Influx endpoints. This is needed as some clients use the health endpoint to determine if the server is healthy and ready for data ingestion. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6653) for the details.
 * FEATURE: [vmctl](https://docs.victoriametrics.com/vmctl/): add `--vm-backoff-retries`, `--vm-backoff-factor`, `--vm-backoff-min-duration` and `--vm-native-backoff-retries`, `--vm-native-backoff-factor`, `--vm-native-backoff-min-duration` command-line flags. These flags allow to change backoff policy config for import requests to VictoriaMetrics. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6622).
 * FEATURE: [vmagent](https://docs.victoriametrics.com/vmagent/): allow overriding the `sample_limit` option at [scrape_configs](https://docs.victoriametrics.com/sd_configs/#scrape_configs) when a label `__sample_limit__` is specified for target. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6665). Thanks to @zoglam for the [pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6666).
+* FEATURE: [vmagent](https://docs.victoriametrics.com/vmagent/): reduce memory usage when scraping targets with big response body. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6759).
 
 * BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert): respect HTTP headers defined in [notifier configuration file](https://docs.victoriametrics.com/vmalert/#notifier-configuration-file) for each request to notifiers. Previously, this param was ignored by mistake.
 * BUGFIX: [stream aggregation](https://docs.victoriametrics.com/stream-aggregation/): correctly apply `-streamAggr.dropInputLabels` when global stream deduplication is enabled without `-streamAggr.config`. Previously, `-remoteWrite.streamAggr.dropInputLabels` was used instead.

lib/bytesutil/bytebuffer.go

@@ -65,7 +65,11 @@ func (bb *ByteBuffer) ReadFrom(r io.Reader) (int64, error) {
 	offset := bLen
 	for {
 		if free := len(b) - offset; free < offset {
-			b = slicesutil.SetLength(b, 2*len(b))
+			// grow slice by 30% similar to how Go does this
+			// https://go.googlesource.com/go/+/2dda92ff6f9f07eeb110ecbf0fc2d7a0ddd27f9d
+			// higher growth rates could consume excessive memory when reading big amounts of data.
+			n := 1.3 * float64(len(b))
+			b = slicesutil.SetLength(b, int(n))
 		}
 		n, err := r.Read(b[offset:])
 		offset += n