test/libc_many_writes for examining write batching

This test reveals the patters of the batching of consecutive write
operations on a file-system session. It issues 100 writes of one byte
each, which should ideally result in large batches of operations
submitted to the file-system session at once.

The run script performs the write operations through a chain of two VFS
servers, thereby exercising the write batching of both the libc and the
intermediate VFS server.

Issue #4697

repos/libports/run/libc_many_writes.run

@@ -0,0 +1,86 @@
+build {
+	core lib/ld init timer
+	lib/vfs lib/vfs_audit server/vfs
+	lib/libc lib/posix test/libc_many_writes
+}
+
+create_boot_directory
+
+install_config {
+<config>
+	<parent-provides>
+		<service name="CPU"/>
+		<service name="IRQ"/>
+		<service name="IO_MEM"/>
+		<service name="IO_PORT"/>
+		<service name="LOG"/>
+		<service name="PD"/>
+		<service name="RM"/>
+		<service name="ROM"/>
+	</parent-provides>
+
+	<default-route>
+		<any-service> <parent/> <any-child/> </any-service>
+	</default-route>
+
+	<default caps="128"/>
+
+	<start name="timer">
+		<resource name="RAM" quantum="1M"/>
+		<provides> <service name="Timer"/> </provides>
+	</start>
+
+	<start name="ramfs">
+		<binary name="vfs"/>
+		<resource name="RAM" quantum="4M"/>
+		<provides> <service name="File_system"/> </provides>
+		<config>
+			<vfs>
+				<dir name="ram"> <ram/> </dir>
+				<dir name="audit"> <audit path="ram"/> </dir>
+			</vfs>
+			<default-policy root="/audit" writeable="yes"/>
+		</config>
+	</start>
+
+	<start name="fs">
+		<binary name="vfs"/>
+		<resource name="RAM" quantum="4M"/>
+		<provides> <service name="File_system"/> </provides>
+		<config>
+			<vfs>
+				<dir name="fs"> <fs/> </dir>
+				<dir name="audit"> <audit path="fs"/> </dir>
+			</vfs>
+			<default-policy root="/audit" writeable="yes"/>
+		</config>
+		<route>
+			<service name="File_system"> <child name="ramfs"/> </service>
+			<any-service> <parent/> <any-child/> </any-service>
+		</route>
+	</start>
+
+	<start name="test-libc_many_writes" caps="1000">
+		<resource name="RAM" quantum="4M"/>
+		<config>
+			<vfs>
+				<dir name="rw">  <fs/>  </dir>
+				<dir name="dev"> <log/> </dir>
+			</vfs>
+			<libc stdout="/dev/log" stderr="/dev/log"/>
+			<arg value="test"/>
+		</config>
+		<route>
+			<service name="File_system"> <child name="fs"/> </service>
+			<any-service> <parent/> <any-child/> </any-service>
+		</route>
+	</start>
+</config>
+}
+
+build_boot_image [build_artifacts]
+
+append qemu_args " -nographic  "
+
+run_genode_until "child .* exited with exit value 0.*\n" 20
+

repos/libports/src/test/libc_many_writes/main.c

@@ -0,0 +1,59 @@
+/*
+ * \brief  Libc test stressing the batching of write operations
+ * \author Norman Feske
+ * \date   2022-12-01
+ */
+
+/*
+ * Copyright (C) 2022 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <string.h>
+#include <time.h>
+#include <fcntl.h>
+
+
+static void print_time(char const *msg)
+{
+	struct timespec tp;
+	bzero(&tp, sizeof(tp));
+
+	int res = clock_gettime(CLOCK_MONOTONIC, &tp);
+	if (res != 0) {
+		printf("error: clock_gettime failed (%d)\n", res);
+		exit(-1);
+	}
+
+	printf("%s:  %ld s  %ld ms\n", msg, tp.tv_sec, tp.tv_nsec/1000000);
+}
+
+
+int main(int argc, char **argv)
+{
+	char const * const path = "/rw/data";
+
+	int const fd = open(path, O_CREAT | O_RDWR);
+	if (fd < 0)
+		printf("error: creation of file '%s' failed (%d)\n", path, fd);
+
+	print_time("start");
+
+	for (int i = 0; i < 100; i++) {
+		printf("write\n");
+		unsigned char c = i & 0xffu;
+		write(fd, &c, 1);
+	}
+
+	print_time("end");
+
+	close(fd);
+
+	printf("exiting\n");
+	return 0;
+}

repos/libports/src/test/libc_many_writes/target.mk

@@ -0,0 +1,3 @@
+TARGET = test-libc_many_writes
+SRC_C  = main.c
+LIBS   = posix