runtime: reduce allocations done in runtime.used_memory/0 on linux (#24901)

Author: spytheman

vlib/runtime/used_memory_linux.c.v

@@ -3,17 +3,44 @@ module runtime
 import os
 
 // used_memory retrieves the current physical memory usage of the process.
+@[manualfree]
 pub fn used_memory() !u64 {
-	file := '/proc/self/status'
-	content := os.read_file(file) or { return 0 }
-	for line in content.split_into_lines() {
-		if line.starts_with('VmRSS:') {
-			parts := line.split(':')
-			if parts.len > 1 {
-				value := parts[1].trim_space().replace('kB', '').trim_space()
-				return value.u64() * 1024 // Convert to bytes
+	buffer := [1024]u8{}
+	pc := unsafe { &buffer[0] }
+	// see https://man7.org/linux/man-pages/man5/proc_pid_stat.5.html for a detailed description of the format.
+	// Here is an example:
+	// 699989 (test_program) R 178038 699989 178038 34816 699989 0 102 0 0 0 0 0 0 0 20 0 1 0 84188763 5726208 129 18446744073709551615 93824992239616 93824992257073 140737488345056 0 0 0 0 0 0 0 0 0 17 0 0 0 0 0 0 93824992275088 93824992276672 93824992280576 140737488346201 140737488346221 140737488346221 140737488351211 0
+	// => read the file in the fixed buffer, search to after the `)` to skip the variable name part, then scan till the `129` RSS value, and then convert it from pages to bytes.
+	mut f := os.open_file('/proc/self/stat', 'r')!
+	defer { f.close() }
+	read := f.read_into_ptr(pc, buffer.len)!
+	if read <= 0 {
+		return error('could not read from /proc/self/stat')
+	}
+	mut c := 0
+	unsafe {
+		// scan to after the process name (`comm`) field:
+		for c < buffer.len && *pc != `)` {
+			pc++
+			c++
+		}
+		mut spaces := 0
+		// the following fields, are space separated numbers, skip past them to the `rss` field:
+		for c < buffer.len && spaces < 22 {
+			if *pc == ` ` {
+				spaces++
 			}
+			pc++
+			c++
+		}
+		mut ndigits := 0
+		// scan till the end of the `rss` field:
+		for c < buffer.len && *(pc + ndigits) != ` ` {
+			ndigits++
+			c++
 		}
+		rss_pages := pc.vstring_with_len(ndigits).u64()
+		page_size := C.sysconf(C._SC_PAGESIZE)
+		return u64(rss_pages * page_size)
 	}
-	return 0
 }