process_metrics_linux.go: avoid calling os.Getpagesize() with every w…

…riteProcessMetrics() call

It is enough to detect OS page size at the start and then use the detected value.
This removes syscall overhead from every writeProcessMetrics() call.

This is a follow-up for 7b14975
Updates #76
Updates VictoriaMetrics/VictoriaMetrics#6457

process_metrics_linux.go

@@ -16,6 +16,11 @@ import (
 // See https://github.com/prometheus/procfs/blob/a4ac0826abceb44c40fc71daed2b301db498b93e/proc_stat.go#L40 .
 const userHZ = 100
 
+// Different environments may have different page size.
+//
+// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6457
+var pageSizeBytes = uint64(os.Getpagesize())
+
 // See http://man7.org/linux/man-pages/man5/proc.5.html
 type procStat struct {
 	State       byte
@@ -80,7 +85,7 @@ func writeProcessMetrics(w io.Writer) {
 	WriteCounterUint64(w, "process_major_pagefaults_total", uint64(p.Majflt))
 	WriteCounterUint64(w, "process_minor_pagefaults_total", uint64(p.Minflt))
 	WriteGaugeUint64(w, "process_num_threads", uint64(p.NumThreads))
-	WriteGaugeUint64(w, "process_resident_memory_bytes", uint64(p.Rss)*uint64(os.Getpagesize()))
+	WriteGaugeUint64(w, "process_resident_memory_bytes", uint64(p.Rss)*pageSizeBytes)
 	WriteGaugeUint64(w, "process_start_time_seconds", uint64(startTimeSeconds))
 	WriteGaugeUint64(w, "process_virtual_memory_bytes", uint64(p.Vsize))
 	writeProcessMemMetrics(w)