Raise the butler's I/O priority

This likely won't make much difference on modern systems,
since it requires a kernel based I/O scheduler. which is
disabled (set to "none" for NVMe and SSDs).

libs/ardour/butler.cc

@@ -24,6 +24,10 @@
 #include <fcntl.h>
 #include <unistd.h>
 
+#ifdef HAVE_IOPRIO
+#include <sys/syscall.h>
+#endif
+
 #ifndef PLATFORM_WINDOWS
 #include <poll.h>
 #endif
@@ -177,6 +181,13 @@ Butler::thread_work ()
 	bool                disk_work_outstanding = false;
 	RouteList::iterator i;
 
+#ifdef HAVE_IOPRIO
+	// ioprio_set (IOPRIO_WHO_PROCESS, 0 /*calling thread*/, IOPRIO_PRIO_VALUE (IOPRIO_CLASS_RT, 4))
+	if (0 != syscall (SYS_ioprio_set, 1, 0, (1 << 13) | 4)) {
+		warning << _("Cannot set I/O Priority for disk read/write thread") << endmsg;
+	}
+#endif
+
 	while (true) {
 		DEBUG_TRACE (DEBUG::Butler, string_compose ("%1 butler main loop, disk work outstanding ? %2 @ %3\n", DEBUG_THREAD_SELF, disk_work_outstanding, g_get_monotonic_time ()));
 

libs/ardour/wscript

@@ -345,9 +345,19 @@ def configure(conf):
 
     conf.check(header_name='sys/vfs.h', define_name='HAVE_SYS_VFS_H',mandatory=False)
     conf.check(header_name='sys/statvfs.h', define_name='HAVE_SYS_STATVFS_H',mandatory=False)
-
     conf.check(header_name='unistd.h', define_name='HAVE_UNISTD',mandatory=False)
 
+    have_sys_ioprio = conf.check_cc(
+            msg="Checking for 'ioprio_set' syscall support",
+            features  = 'c',
+            mandatory = False,
+            execute   = False,
+            fragment = "#include <sys/syscall.h>\nint main () { syscall(SYS_ioprio_set, 1, 0, 8192); return 0; }")
+
+    if have_sys_ioprio:
+            conf.define('HAVE_IOPRIO', 1)
+            conf.env['HAVE_IOPRIO'] = True
+
     conf.write_config_header('libardour-config.h', remove=False)
 
     # Boost headers

wscript

@@ -1571,6 +1571,7 @@ const char* const ardour_config_info = "\\n\\
     write_config_text('Futex Semaphore',       conf.is_defined('USE_FUTEX_SEMAPHORE'))
     write_config_text('Freedesktop files',     opts.freedesktop)
     write_config_text('G_ENABLE_DEBUG',        opts.gdebug or conf.env['DEBUG'])
+    write_config_text('I/O Priorty Set',       conf.is_defined('HAVE_IOPRIO'))
     write_config_text('Libjack linking',       conf.env['libjack_link'])
     write_config_text('Libjack metadata',      conf.is_defined ('HAVE_JACK_METADATA'))
     write_config_text('Lua Binding Doc',       conf.is_defined('LUABINDINGDOC'))