Mail Archives: cygwin/2009/06/25/10:37:18
On Jun 25 10:05, Christopher Faylor wrote:
> On Thu, Jun 25, 2009 at 12:10:39PM +0200, Corinna Vinschen wrote:
> >On Jun 24 13:17, Andrew Schulman wrote:
> >> Here's an odd one.
> >>
> >> Using openssh 5.2p1-2 with Cygwin 1.7.0-50, when I scp any file, the
> >> progress counter appears to show ridiculously fast transfer rates, e.g.
> >> about 50 MB/s over a 750 KB/s connection, for the first 175 MB or so. After
> >> that the counter settles down to normal speed. Then when the counter
> >> reaches the end, it "hangs" at 100% for the remaining time while the copy
> >> finishes.
> >>
> >> At first I thought that the copy itself was being corrupted in the first
> >> 175 MB, but I'm no longer able to reproduce that. I believe now that the
> >> copy is good and it's only the progress counter that's wrong.
> >>
> >> When I revert to Cygwin 1.7.0-49, this problem doesn't occur.
> >
> >I can reproduce that copying a file via scp from a Windows machine to
> >a Linux box.
> >
> >It looks like the pipes between the local scp and the local ssh are now
> >exchanging the data quicker at the start than the ssh socket can send
> >them to the remote machine. On my XP machine, scp advances quickly by
> >about 260 Megs (hard to tell, maybe it's exaclty 256 Megs for some
> >reason?), then keeps the advance roughly at that value until scp
> >finished. At the end scp is just waiting for ssh which still has to
> >send the 256/260 Megs of data.
> >
> >This is really weird, given that Cygwin does not create such a big
> >buffer for the pipe. Consequentially Task Manager claims that the
> >memory is neither taken by scp, nor by ssh. Both processes have normal
> >VM sizes < 10 Megs. Per Task manager the memory is paged Kernel Memory.
> >A strange side effect is that the entire time taken by the data
> >transmission is longer than with -49, by almost exactly the time it
> >takes to empty the big kernel cache.
> >
> >Puzzeling.
>
> Is ssh using non-blocking pipes opened for write? Until a week or two
> ago, Cygwin didn't support those and treated the non-blocking write as a
> blocking write.
scp switches the pipes to non-blocking and then tries to do blocking io
on its own, using the poll() function. It calls a function called scpio
which basically work like this:
scpio (io_function, fd, buf, size)
{
for (offset = 0; offset < size;) {
r = io_function (fd, buf + offset, size - offset);
[...]
if (r < 0) {
if (errno == EINTR)
continue;
if (errno == EAGAIN || errno == EWOULDBLOCK) {
poll (fd, 1, -1) /* Use poll() for blocking */
continue;
}
[...]
}
offset += r;
}
}
Looks like scp now stumbles over the pipe select() implementation.
Btw., scp knows a `-l bandwidth' parameter to allow to specify a maximum
throughput. If you set this to a value which corresponds to the maximum
throughput of the sending socket in ssh, then you can "optimize" the
problem away and you're back to the old timings.
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
Red Hat
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