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From Gregory Shimansky <gshiman...@apache.org>
Subject Re: [drlvm] Use of SIGUSR2
Date Sun, 01 Mar 2009 18:37:37 GMT
On 1 March 2009 Gregory Shimansky wrote:
> On 1 March 2009 Mark Hindess wrote:
> > The latest issue on the "Problems with NIO" thread (message dated Wed,
> > 25 Feb 2009 23:09:49 +0100) seems to another case of a syscall being
> > interrupted by an internal SIGUSR2 and reporting an error that needs to
> > be caught and the signal retried.  There are already several cases where
> > the code "works around" this issue.  And *every* occurence of affected
> > syscalls[0] needs to be wrapped in this logic.
> >
> > The IBM VM doesn't do this kind of wrapping and I assume neither does
> > Sun's since all signals they receive are external and thus raising
> > the exception is the required behaviour.  I can't help thinking that
> > replacing the use of SIGUSR2 would be an easier option long term - and
> > better for performance - than wrapping every syscall.
>
> Actually if you strace Sun or Jrockit you would see tons of SIGUSR[1|2]
> (different VMs use different numbers) signals. There is even a special
> switch -XX:+UseAltSigs which allows Sun's VM to use signals other than
> SIGUSR for their internal use [1].
>
> I found other pages about signal usages by Sun's and BEA VMs [2][3][4].

BTW BEA's page explicitly states that it is necessary to wrap all of the 
syscalls with retry if there is EINTR. And since BEA's VM uses Sun's classlib 
it means that all of the syscalls in Sun's classlib are wrapper.

> > From a selfish classlib perspective, it is wrong to fix these issues in
> > classlib since wrapping syscalls imposes a performance penalty on all
> > VMs (such as Jikes and IBM's) even though they do not need this extra
> > code.  It certainly makes the code harder to read, write and maintain.
> > So it would be nice to "fix" it in DRLVM.
> >
> > Can someone explain why DRLVM uses SIGUSR2?  And what alternatives are
> > there?  Could we not use signals?  Other VMs seem to manage without
> > them?  How?  Could we mask the signal when entering native oode?
> >
> > Regards,
> >  Mark.
> >
> > [0] The DRLVM code does use the SA_RESTART flag so some syscalls are
> > automatically restarted but many calls are not.  On Linux[1], the most
> > important for us are epoll_wait(2), epoll_pwait(2), poll(2), ppoll(2),
> > select(2), and pselect(2).  For precise details, see "Interruption of
> > System Calls and Library Functions by Signal Handlers" in signal(7).
> >
> > [1] The signal(7) man page also says "The details vary across Unix
> > systems" which implies that the calls needing wrapping will be different
> > across different unix systems making porting classlib for DRLVM on
> > non-Linux systems even more difficult.  (Of course, this will only be an
> > issue if DRLVM ever gets ported to anything else.)
>
> [1] http://java.sun.com/j2se/1.5.0/docs/tooldocs/solaris/java.html
> [2] http://java.sun.com/javase/6/webnotes/trouble/TSG-VM/html/signals.html
> [3] http://edocs.bea.com/jrockit/geninfo/devapps/codeprac.html#wp1005153
> [4] http://java.sun.com/j2se/1.5.0/docs/guide/vm/signal-chaining.html



-- 
Gregory

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