[influxd] Use a sync.Pool to reuse gzip.Writer across requests

This brings alloc_space down from ~20200M to ~10700M in a run of
go test ./cmd/influxd/run -bench=Server -memprofile=mem.out -run='^$'

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### Features
 
 - [#7776](https://github.com/influxdata/influxdb/issues/7776): Add system information to /debug/vars.
+- [#7948](https://github.com/influxdata/influxdb/pull/7948): Reduce memory allocations by reusing gzip.Writers across requests
 
 ## v1.2.1 [unreleased]
 

services/httpd/handler.go

@@ -8,6 +8,7 @@ import (
 	"expvar"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"log"
 	"net/http"
 	"net/http/pprof"
@@ -15,6 +16,7 @@ import (
 	"runtime/debug"
 	"strconv"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -1087,13 +1089,23 @@ func gzipFilter(inner http.Handler) http.Handler {
 			inner.ServeHTTP(w, r)
 			return
 		}
-		gz := gzip.NewWriter(w)
-		defer gz.Close()
+		gz := gzipWriterPool.Get().(*gzip.Writer)
+		gz.Reset(w)
+		defer func() {
+			gz.Close()
+			gzipWriterPool.Put(gz)
+		}()
 		gzw := gzipResponseWriter{Writer: gz, ResponseWriter: w}
 		inner.ServeHTTP(gzw, r)
 	})
 }
 
+var gzipWriterPool = sync.Pool{
+	New: func() interface{} {
+		return gzip.NewWriter(ioutil.Discard)
+	},
+}
+
 // cors responds to incoming requests and adds the appropriate cors headers
 // TODO: corylanou: add the ability to configure this in our config
 func cors(inner http.Handler) http.Handler {