From dd6d8b775490db422595659bad9bd5bbb4360247 Mon Sep 17 00:00:00 2001
From: Florian Lehner <florianl@users.noreply.github.com>
Date: Thu, 23 Nov 2023 10:48:47 +0100
Subject: [PATCH] metric/system: preallocate arrays (#116)

## What does this PR do?

Whenever possible, preallocate array to the correct size instead of
letting arrays "grow". This reduces the load on the Garbage Collector
and improves memory usage efficiency.

## Checklist

- [x] My code follows the style guidelines of this project
- [ ] ~~I have commented my code, particularly in hard-to-understand
areas~~
- [ ] ~~I have added tests that prove my fix is effective or that my
feature works~~
- [ ] ~~I have added an entry in `CHANGELOG.md`~~

Signed-off-by: Florian Lehner <florian.lehner@elastic.co>
---
 metric/system/diskio/diskstat_windows_helper.go | 4 ++--
 metric/system/process/process.go                | 4 ++--
 metric/system/process/process_darwin.go         | 6 +++---
 metric/system/process/process_linux_common.go   | 4 ++--
 metric/system/process/process_windows.go        | 4 ++--
 5 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/metric/system/diskio/diskstat_windows_helper.go b/metric/system/diskio/diskstat_windows_helper.go
index 370b1fd7ef..9baf94376e 100644
--- a/metric/system/diskio/diskstat_windows_helper.go
+++ b/metric/system/diskio/diskstat_windows_helper.go
@@ -59,7 +59,7 @@ func ioCounters(names ...string) (map[string]disk.IOCountersStat, error) {
 	if err != nil || len(logicalDisks) == 0 {
 		return nil, err
 	}
-	ret := make(map[string]disk.IOCountersStat)
+	ret := make(map[string]disk.IOCountersStat, len(logicalDisks))
 	for _, drive := range logicalDisks {
 		// not get _Total or Harddrive
 		if len(drive.Name) > 3 {
@@ -158,7 +158,7 @@ func getLogicalDriveStrings() ([]logicalDrive, error) {
 		}
 		return nil, err
 	}
-	var logicalDrives []logicalDrive
+	logicalDrives := make([]logicalDrive, 0, len(lpBuffer))
 	for _, v := range lpBuffer {
 		if v >= 65 && v <= 90 {
 			s := string(v)
diff --git a/metric/system/process/process.go b/metric/system/process/process.go
index 0b1b80d75d..0bdc05b061 100644
--- a/metric/system/process/process.go
+++ b/metric/system/process/process.go
@@ -115,8 +115,8 @@ func (procStats *Stats) Get() ([]mapstr.M, []mapstr.M, error) {
 	}
 
 	// Format the list to the MapStr type used by the outputs
-	var procs []mapstr.M
-	var rootEvents []mapstr.M
+	procs := make([]mapstr.M, 0, len(plist))
+	rootEvents := make([]mapstr.M, 0, len(plist))
 
 	for _, process := range plist {
 		process := process
diff --git a/metric/system/process/process_darwin.go b/metric/system/process/process_darwin.go
index 950d9734b6..5c17cc2275 100644
--- a/metric/system/process/process_darwin.go
+++ b/metric/system/process/process_darwin.go
@@ -64,8 +64,8 @@ func (procStats *Stats) FetchPids() (ProcsMap, []ProcState, error) {
 
 	bbuf := bytes.NewBuffer(buf)
 
-	procMap := make(ProcsMap, 0)
-	var plist []ProcState
+	procMap := make(ProcsMap, num)
+	plist := make([]ProcState, 0, num)
 
 	for i := 0; i < num; i++ {
 		if err := binary.Read(bbuf, binary.LittleEndian, &pid); err != nil {
@@ -193,7 +193,7 @@ func getProcArgs(pid int, filter func(string) bool) ([]string, string, mapstr.M,
 	}
 
 	// read CLI args
-	var argv []string
+	argv := make([]string, 0, argc)
 	for i := 0; i < int(argc); i++ {
 		arg, err := bbuf.ReadBytes(0)
 		if err == io.EOF {
diff --git a/metric/system/process/process_linux_common.go b/metric/system/process/process_linux_common.go
index 40346a4941..e5a4a498ee 100644
--- a/metric/system/process/process_linux_common.go
+++ b/metric/system/process/process_linux_common.go
@@ -58,8 +58,8 @@ func (procStats *Stats) FetchPids() (ProcsMap, []ProcState, error) {
 		return nil, nil, fmt.Errorf("error reading directory names: %w", err)
 	}
 
-	procMap := make(ProcsMap)
-	var plist []ProcState
+	procMap := make(ProcsMap, len(names))
+	plist := make([]ProcState, 0, len(names))
 
 	// Iterate over the directory, fetch just enough info so we can filter based on user input.
 	logger := logp.L()
diff --git a/metric/system/process/process_windows.go b/metric/system/process/process_windows.go
index dbd116121d..981ee2a1a8 100644
--- a/metric/system/process/process_windows.go
+++ b/metric/system/process/process_windows.go
@@ -38,8 +38,8 @@ func (procStats *Stats) FetchPids() (ProcsMap, []ProcState, error) {
 		return nil, nil, fmt.Errorf("EnumProcesses failed: %w", err)
 	}
 
-	procMap := make(ProcsMap, 0)
-	var plist []ProcState
+	procMap := make(ProcsMap, len(pids))
+	plist := make([]ProcState, 0, len(pids))
 	// This is probably the only implementation that doesn't benefit from our
 	// little fillPid callback system. We'll need to iterate over everything
 	// manually.