| Subject: | Child process priority not inherited due to hardcoded priority |
In Windows there is limited process priority inheritance. If the parent
priority level is 'below normal' or 'low' then child processes will by
default inherit the priority level. In any other case the child will by
default run with 'normal' priority.
Documentation: http://msdn.microsoft.com/en-us/library/ms682425(VS.85).aspx
"If none of the priority class flags is specified, the priority class
defaults to NORMAL_PRIORITY_CLASS unless the priority class of the
creating process is IDLE_PRIORITY_CLASS or BELOW_NORMAL_PRIORITY_CLASS.
In this case, the child process receives the default priority class of
the calling process."
(IDLE is the same as what is elsewhere called 'low'.)
However, in IPC::Run this is not working because the process is always
created with 'normal' priority. This is controlled by
lib/Run/Win32Helper.pm which in 0.82 on line 455 reads:
Win32::Process::Create(
$process,
$cmd->[0],
$cmd_line,
1, ## Inherit handles
NORMAL_PRIORITY_CLASS,
".",
) or croak "$!: Win32::Process::Create()";
By specifying 'NORMAL_PRIORITY_CLASS' in Win32::Process::Create()'s
$cflags option, children will always start with 'normal' priority.
Instead, change that field to 0, e.g.:
Win32::Process::Create(
$process,
$cmd->[0],
$cmd_line,
1, ## Inherit handles
0,
".",
) or croak "$!: Win32::Process::Create()";
With this change, IPC::Run will behave the same in the common case and
children will still run at 'normal' priority in most cases. However if
the parent priority has been changed to 'below normal' or 'low' then the
child will inherit this automatically. I believe this behavior is more
correct than hardcoding the priority. It is certainly what I was
expecting to happen.
Even better would be to expose an option to be able to set the child
process priority, but that is a much more complicated change.