Re: rsync over ssh hang issue understood

[Darryl Miles <darryl-mailinglists at netbauds dot net>]

Lev Bishop wrote:
> On 7/7/06, Darryl Miles wrote:
> > Dave Korn wrote:
> > > On 07 July 2006 01:31, Darryl Miles wrote:
> > The underlying socket is still being used in blocking mode.
>
> Socket?? What is this socket?

Sorry s/socket/pipe/.


> > Which means
> > when we write write 1024 bytes of data but only one PIPE buffer is free
> > (ulimit -p) a POSIX kernel would allow:
>
> What is "one PIPE buffer (ulimit -p)"?

See below...


> > Non-blocking:
> >
> > write(fd, buffer, 1024) = 512
> > write(fd, &buffer[512], 512) = -1 (EAGAIN)
> >
> > Blocking:
> >
> > write(fd, buffer, 1024) = 512
> > write(fd, &buffer[512], 512) = [blocking occurs until] 512
>
> SUSv3 does not permit this behaviour. It is quite clear on this point:
> "If the O_NONBLOCK flag is clear, a write request may cause the thread
> to block, but on normal completion it shall return nbyte."

Okay this is fine too.  I believe the applications which we are seeing 
problems with are using non-blocking mode.



> > I would guess under WIN32 we end up with fhandler.cc:raw_write() doing:
> >
> > WriteFile(hPipe, buffer, 1024, &writtenLen, 0) = [blocking occurs until]
> > TRUE (writtenLen=1024)
>
> This gives the SUSv3-mandated behaviour for blocking pipes (modulo
> signal-interruptibility).

For POSIX blocking mode yes of course, but for POSIX non-blocking mode 
we are relying on the information coming back from 
NtQueryInformationFile to indicate that our WriteFile() won't block 
providing our writes are <= PIPE_BUF length.



> What does "simulate 'ulimit -p'" mean? ulimit -p works just fine on cygwin:
> $ ulimit -p
> 8

I had a complete misunderstanding on that, I was thinking the 
application would tweak the kernel buffering between processes.  But 
this appears to be just a reporting mechanism for PIPE_BUF size at runtime.



What was the reason the existing code in CVS for select.cc was disabled?

Maybe it would help to better understand the reasons/cases where the 
disabled code failed.

Darryl

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