Implement optimized RSS monitor based on /proc/[pid]/statm

The goal of this change is to relieve pressure on forkExec mutex on platforms where this is supported.

Changelog: performance

3 files changed, 55 insertions, 10 deletions

diff --git a/internal/ps/ps.go b/internal/ps/ps.go
index 1115547ac..c3613617c 100644
--- a/internal/ps/ps.go
+++ b/internal/ps/ps.go
@@ -16,16 +16,6 @@ func Exec(pid int, keywords string) (string, error) {
 	return strings.TrimSpace(string(out)), nil
 }
 
-// RSS invokes ps -o rss= -p pid and returns its output
-func RSS(pid int) (int, error) {
-	rss, err := Exec(pid, "rss=")
-	if err != nil {
-		return 0, err
-	}
-
-	return strconv.Atoi(rss)
-}
-
 // Comm invokes ps -o comm= -p pid and returns its output
 func Comm(pid int) (string, error) {
 	return Exec(pid, "comm=")
diff --git a/internal/ps/rss.go b/internal/ps/rss.go
new file mode 100644
index 000000000..f6a02abb2
--- /dev/null
+++ b/internal/ps/rss.go
@@ -0,0 +1,17 @@
+// +build !linux
+
+package ps
+
+import (
+	"strconv"
+)
+
+// RSS invokes ps -o rss= -p pid and returns its output
+func RSS(pid int) (int, error) {
+	rss, err := Exec(pid, "rss=")
+	if err != nil {
+		return 0, err
+	}
+
+	return strconv.Atoi(rss)
+}
diff --git a/internal/ps/rss_linux.go b/internal/ps/rss_linux.go
new file mode 100644
index 000000000..650fe03a8
--- /dev/null
+++ b/internal/ps/rss_linux.go
@@ -0,0 +1,38 @@
+package ps
+
+import (
+	"fmt"
+	"os"
+)
+
+var pageSize = os.Getpagesize()
+
+// https://gitlab.com/procps-ng/procps/-/blob/37f106029975e3045b0cd779525d14c55d24b74e/proc/readproc.h#L51
+// https://man7.org/linux/man-pages/man5/proc.5.html
+type statm struct {
+	size, resident, shared, text, lib, data, dt int
+}
+
+// RSS returns the RSS of a process, in kB
+func RSS(pid int) (int, error) {
+	file, err := os.Open(fmt.Sprintf("/proc/%d/statm", pid))
+	if err != nil {
+		return 0, err
+	}
+	defer file.Close()
+
+	s := statm{}
+
+	// unit for each of these is pages
+	// https://gitlab.com/procps-ng/procps/-/blob/37f106029975e3045b0cd779525d14c55d24b74e/proc/readproc.c#L660
+	_, err = fmt.Fscanf(file, "%d %d %d %d %d %d %d",
+		&s.size, &s.resident, &s.shared,
+		&s.text, &s.lib, &s.data, &s.dt)
+	if err != nil {
+		return 0, err
+	}
+
+	rssKbytes := (s.resident * pageSize) / 1024
+
+	return rssKbytes, nil
+}