Re: Network compatibility and performance

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Sat, 12 Aug 2006, Ben Greear wrote:

> linux-os (Dick Johnson) wrote:
>> Hello,
>>
>> Network throughput is seriously defective with linux-2.6.16.24
>> if the length given to 'write()' is a large number.
>>
>> Given this code on a connected socket........
>>
>> //-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
>> //
>> //  Copyright(c)  2005  Analogic Corporation    (rjohnson@analogic.com)
>> //
>> //  This program may be distributed under the GNU Public License
>> //  version 2, as published by the Free Software Foundation, Inc.,
>> //  59 Temple Place, Suite 330 Boston, MA, 02111.
>> //
>> //-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
>>
>> #include <stdio.h>
>> #include <unistd.h>
>> #include <stdlib.h>
>> #include <stdint.h>
>> #include <signal.h>
>> #include <string.h>
>> #include <stdarg.h>
>> #include <errno.h>
>> #include <fcntl.h>
>> #include <netinet/in.h>
>> #include <netinet/tcp.h>
>> #include <sys/poll.h>
>>
>> #define BUF_LEN 0x1000
>> #define FAIL -1
>>
>> //-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
>> //
>> //   This sends a message that could exceed the size of the network buffers.
>> //   It returns 0 if everything went okay, and FAIL if not.
>> //
>> int32_t sender(int32_t fd, void *buf, size_t len)
>> {
>>      int32_t ret_val;
>>      uint8_t *cp;
>>      cp = (uint8_t *) buf;
>>      while(len) {
>>          if((ret_val = write(fd, cp, MIN(len, BUF_LEN))) == FAIL) {
>>              if(errno == EAGAIN)
>>                  continue;
>>              return ret_val;
>>          }
>>          len -= ret_val;
>>          cp  += ret_val;
>>      }
>>      return 0;
>> }
>>
>> It used to work quite well with:
>>
>>      while(len) {
>>          if((ret_val = write(fd, cp, len)) == FAIL) {
>>                  return ret_val;
>>          }
>>          len -= ret_val;
>>          cp  += ret_val;
>>      }
>>
>> The network socket layer would return the amount of bytes
>> actually sent and the code would walk its way up through the
>> buffer. This was the expected behavior for many years.
>>
>> Then after about Linux-2.6.8, I needed to do:
>>
>>      while(len) {
>>          if((ret_val = write(fd, cp, len)) == FAIL) {
>>              if(errno == EAGAIN)
>>                  continue;
>>              return ret_val;
>>          }
>>          len -= ret_val;
>>          cp  += ret_val;
>>      }
>>
>> This was because Linux would claim to run out of resources
>> even though there was nothing else running on the system.
>>
>> Now at Linux-2.6.16.24, the code needed to be further modified
>> to:
>>      while(len) {
>>          if((ret_val = write(fd, cp, MIN(len, 0x1000))) == FAIL) {
>>              if(errno == EAGAIN)
>>                  continue;
>>              return ret_val;
>>          }
>>          len -= ret_val;
>>          cp  += ret_val;
>>      }
>
> In the case where you are getting EAGAIN, this is a busy-spin.  You
> might want to sleep in a select() or similar call as soon as you get
> EAGAIN on this socket..or go off and do other work while the OS clears
> out the send queue.
>
> Also, from your description, this code should return 0 on success.  It
> is returning 'ret_val' instead, which should be > 0.

No it will return FAIL (-1) or an error and 0 (the bottom of the procedure)
if the whole things went. It is mandatory that the whole thing goes
so this procedure should handle any intermediate actions.

Upon your advice, I may try to add select() although, on a write it
seems to be putting in user-space something that used to be handled
quite well in the kernel. I don't think the user should really care
about the kernel internals, whether or not the kernel happens to have
a buffer available.

>
> I have no idea why you need to add the MIN() logic..and that seems like
> something that should not be required.
>

It seems that some code 'thinks' that a large buffer of data is
an error and won't even try to send some anymore.


>> ... or else it would spin <forever> returning 0 with no errno set.
>> In all cases, these problems exist when 'len' is a large value, perhaps
>> 0x01000000, much greater than Linux could ever find an available
>> buffer for. Linux used to send what it could. Now it will just fail to
>> send anything at all, returning 0 if it 'feels' like it doesn't want
>> to bother. This is not good. With the hacked code, the data throughput
>> is about 100,000 bytes per second on a dedicated link. The previous
>> code ran 112,000 bytes per second. Once the 'errno' happens, the
>> network stumbles to a measley 12,000 bytes per second. This
>> breaks our applications.
>
> Even 112kbps sucks on a decent network.  What is the speed of your
> network, what protocol are you using, if tcp, what is the latency
> of your network?
>

The network is a single wire about 8 feet long, connecting Intel gigibit
links on two identical computers (crossover cable). This link is TCP.
For high-speed data, I use UDP and I get a higher throughput because
there is no handshake. Thew latency is the latency of Linux. BTW, it's
only a gigaBIT link, you can divide that by 8 for gigabytes. I don't
know the actual bit-rate on the wires, if we assume 1GHz, the byte-rate
is only 125,000 bytes per second. Being able to use 89.6 percent of
that isn't bad at all.


> Ben
>
>
>

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.62 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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