retrieval: handle GZIP compression ourselves

The automatic GZIP handling of net/http does not preserve
buffers across requests and thus generates a lot of garbage.
We handle GZIP ourselves to circumvent this.t

retrieval/scrape.go

@@ -14,12 +14,16 @@
 package retrieval
 
 import (
+	"bufio"
 	"bytes"
+	"compress/gzip"
 	"fmt"
 	"io"
 	"net/http"
+	"reflect"
 	"sync"
 	"time"
+	"unsafe"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/log"
@@ -347,35 +351,30 @@ type scraper interface {
 // targetScraper implements the scraper interface for a target.
 type targetScraper struct {
 	*Target
-	client *http.Client
-}
-
-const acceptHeader = `application/vnd.google.protobuf;proto=io.prometheus.client.MetricFamily;encoding=delimited;q=0.7,text/plain;version=0.0.4;q=0.3,*/*;q=0.1`
 
-var scrapeBufPool = sync.Pool{}
+	client *http.Client
+	req    *http.Request
 
-func getScrapeBuf() []byte {
+	gzipr *gzip.Reader
-	b := scrapeBufPool.Get()
+	buf   *bufio.Reader
-	if b == nil {
-		return make([]byte, 0, 8192)
-	}
-	return b.([]byte)
 }
 
-func putScrapeBuf(b []byte) {
+const acceptHeader = `application/vnd.google.protobuf;proto=io.prometheus.client.MetricFamily;encoding=delimited;q=0.7,text/plain;version=0.0.4;q=0.3,*/*;q=0.1`
-	b = b[:0]
-	scrapeBufPool.Put(b)
-}
 
 func (s *targetScraper) scrape(ctx context.Context, w io.Writer) error {
+	if s.req == nil {
 		req, err := http.NewRequest("GET", s.URL().String(), nil)
 		if err != nil {
 			return err
 		}
 		// Disable accept header to always negotiate for text format.
 		// req.Header.Add("Accept", acceptHeader)
+		req.Header.Add("Accept-Encoding", "gzip")
+
+		s.req = req
+	}
 
-	resp, err := ctxhttp.Do(ctx, s.client, req)
+	resp, err := ctxhttp.Do(ctx, s.client, s.req)
 	if err != nil {
 		return err
 	}
@@ -385,10 +384,27 @@ func (s *targetScraper) scrape(ctx context.Context, w io.Writer) error {
 		return fmt.Errorf("server returned HTTP status %s", resp.Status)
 	}
 
+	if resp.Header.Get("Content-Encoding") != "gzip" {
 		_, err = io.Copy(w, resp.Body)
 		return err
 	}
 
+	if s.gzipr == nil {
+		s.buf = bufio.NewReader(resp.Body)
+		s.gzipr, err = gzip.NewReader(s.buf)
+		if err != nil {
+			return err
+		}
+	} else {
+		s.buf.Reset(resp.Body)
+		s.gzipr.Reset(s.buf)
+	}
+
+	_, err = io.Copy(w, s.gzipr)
+	s.gzipr.Close()
+	return err
+}
+
 // A loop can run and be stopped again. It must not be reused after it was stopped.
 type loop interface {
 	run(interval, timeout time.Duration, errc chan<- error)
@@ -436,7 +452,10 @@ func (sl *scrapeLoop) run(interval, timeout time.Duration, errc chan<- error) {
 	ticker := time.NewTicker(interval)
 	defer ticker.Stop()
 
+	buf := bytes.NewBuffer(make([]byte, 0, 16000))
+
 	for {
+		buf.Reset()
 		select {
 		case <-sl.ctx.Done():
 			return
@@ -456,8 +475,6 @@ func (sl *scrapeLoop) run(interval, timeout time.Duration, errc chan<- error) {
 			)
 		}
 
-		buf := bytes.NewBuffer(getScrapeBuf())
-
 		err := sl.scraper.scrape(scrapeCtx, buf)
 		if err == nil {
 			b := buf.Bytes()
@@ -465,7 +482,6 @@ func (sl *scrapeLoop) run(interval, timeout time.Duration, errc chan<- error) {
 			if total, added, err = sl.append(b, start); err != nil {
 				log.With("err", err).Error("append failed")
 			}
-			putScrapeBuf(b)
 		} else if errc != nil {
 			errc <- err
 		}
@@ -524,7 +540,7 @@ loop:
 			t = *tp
 		}
 
-		mets := string(met)
+		mets := yoloString(met)
 		ref, ok := sl.cache[mets]
 		if ok {
 			switch err = app.AddFast(ref, t, v); err {
@@ -550,6 +566,8 @@ loop:
 			default:
 				break loop
 			}
+			// Allocate a real string.
+			mets = string(met)
 			sl.cache[mets] = ref
 		}
 		added++
@@ -567,6 +585,16 @@ loop:
 	return total, added, nil
 }
 
+func yoloString(b []byte) string {
+	sh := (*reflect.SliceHeader)(unsafe.Pointer(&b))
+
+	h := reflect.StringHeader{
+		Data: sh.Data,
+		Len:  sh.Len,
+	}
+	return *((*string)(unsafe.Pointer(&h)))
+}
+
 func (sl *scrapeLoop) report(start time.Time, duration time.Duration, scraped, appended int, err error) error {
 	sl.scraper.report(start, duration, err)
 

retrieval/target.go

@@ -81,6 +81,7 @@ func NewHTTPClient(cfg config.HTTPClientConfig) (*http.Client, error) {
 		Proxy:              http.ProxyURL(cfg.ProxyURL.URL),
 		MaxIdleConns:       10000,
 		TLSClientConfig:    tlsConfig,
+		DisableCompression: true,
 	}
 
 	// If a bearer token is provided, create a round tripper that will set the