rsync 2.5.5: HP-UX 11.11 vs. Linux

[Bojan Smojver]

I'm observing a rather strange behaviour when attempting to transfer files from
a Linux box (RH 7.3, kernel 2.4.19-rc1) to HP-UX 11.11 box. Both are running
rsync 2.5.5.

Everything starts off really nice an quick, but the transfer then slows down to
a crawl. On 100 Mb/s network I'm getting a few hunder bytes to a few hundred
kilobytes per second. Other programs, like RCP and FTP, do not seem to have that
problem.

After running strace on Linux side and tusc on HP-UX side, I'm seing this:

Linux-------------------

select(1, NULL, [0], NULL, {240, 0})    = 1 (out [0], left {207, 980000})
write(0, "\374\17\0\7TORAGE              73  74  "..., 4096) = 4096

Linux-------------------

HP-UX-------------------

read(3, "    5 4     6 9     1       1   ".., 4092) ...... = 3434
time(NULL) ............................................... = 1027926587
select(4, 0x7f7f3010, NULL, NULL, 0x7f7f3110) ............ [sleeping]

HP-UX-------------------

It seems as if Linux belives it wrote out 4096 characters and HP-UX reads much
less than that. When things go 'out of sync' like that, everything becomes very
slow. While everthing's fast, the behaviour on HP-UX side is very uniform: read
4 bytes (I'm guessing the 32-bit size of the packet?) and then read 4092 bytes
(payload?). There is an odd exception here and there, I'm guessing due to file ends.

Any ideas why something like this would be happening?

Bojan