[clue-tech] MySQL server reply packets delayed/lost under network
congestion?
Jim Ockers
ockers at ockers.net
Tue Nov 24 13:59:04 MST 2009
OK this is complicated enough that I think I have to top-post. We have
more information so here's what we've found so far.
Recap: We are using MySQL over a 921600 bps serial connection with Linux
pppd on one side and Windows RAS on the other side. The MySQL server is
on the Linux side and we are using Windows XP as the client. The ppp
interface txqueuelen is 100 and the MTU is 1500, all other PPP options
are standard or default. Everything works fine until we start a high
bandwidth download where the Windows system retrieves something from the
Internet, in which the Linux server has to route the packets. It does
not do NAT. The internet-connected interface is eth0, so the big
download comes in the Linux server on eth0 and goes through netfilter
and routing and goes out the PPP interface. The big download can be
youtube HD streaming, a big FTP file, a big image or document retrieved
via HTTP, or whatever.
It does not matter how big the file is - if there are no dropped
packets, then the MySQL initial banner response is delayed until the
completion of the download, which could be hundreds of megabytes. We
are asking for the banner response by typing "telnet 10.0.0.2 3306" and
are expecting the MySQL banner packet to show up right away, but it
doesn't. There is nothing unusual in the SYN or ACK packets.
The problem is that ALL MySQL response packets generated by mysqld seem
to be not inserted into the Linux kernel network queue discipline send
buffer until after the transfer is finished, or until there is a dropped
packet which causes a momentary delay due to TCP retransmit somewhere in
the middle of the big download. The SYN, SYN-ACK, ACK 3 way handshake
works immediately because those are generated by the OS, but the MySQL
banner is delayed, as are all other MySQL query responses including
"select 1". All other services including httpd (apache), vsftpd, smbd,
etc. work as expected and the service banner (or whatever data) is
transmitted within 100ms or so of the completion of the 3 way
handshake. The timing of packets as shown in a PCAP packet capture is
more or less identical on both sender and receiver.
1. This problem exists on MySQL 4 on 2.4.28 kernel (Red Hat 7.2 base).
2. The exact same problem exists on MySQL 5 on 2.6.18 kernel (CentOS 5.2
base).
3. The problem is masked when we change the ppp interface txqueuelen to
3, thus causing dropped packets inside the Linux kernel and TCP
retransmits which slows down the connection.
4. The problem is masked when we enable tc (traffic shaping) on the 2.6
kernel and set an absolute bandwidth limit on the PPP interface. No
specific shaping rules or prioritization is required, all we have to do
to change the queueing behavior is to enable a bandwidth limit.
5. The problem is masked when we change the Windows serial port RX FIFO
to some value that causes UART overruns and thus dropped packets and TCP
retransmits.
6. The problem is masked if we make the PPP MTU smaller than 1500.
7. If we FTP the file directly from the Linux server, the problem does
not exist. Only downloads from "the other side" of the Linux server
cause this problem.
8. MySQL sets a non-zero TOS in the packet TCP header. However we
mangled it with iptables and this had no effect on the queue discipline.
The problem seems to be related to MySQL's ability (in all versions of
MySQL) to write packets to the transmit queue of the Linux kernel. We
think it is something to do with the queue discipline for sockets or
socket buffers on a PPP interface, because when we set the interface
bandwidth with tc on the 2.6 kernel, the queue discipline seems to
change behavior. We are pretty sure the problem does not exist on
ethernet interfaces, and since there is nothing about this in any google
search we must be the only people in the world doing MySQL queries over
a Linux PPP interface.
Does anyone have any ideas what we could do to make MySQL behave like
all the other Linux services and write its packets to the network even
when the interface is busy? Or else does anyone have any ideas what we
could change in the networking in the kernel to make it less unfriendly
to MySQL packets when the PPP interface is busy with throughput-traffic?
Thanks,
Jim
chris fedde wrote:
> "In for a penny, in for a pound." As they say.
>
> I'm sure you've turned on all the mysqld logging available. The
> --debug option turns on lots of diagnostics but unfortunately the
> typical distro packages don't have the --debug features enabled.
> you might glean some information running strace on the mysqld running
> process while the connection problem happens.
>
> That the problem is traffic related might be an indication of some
> priority queueing behaviour in an intermediate router or firewall
> device. Or perhaps the network has some asymmetric bandwidth feature
> that expresses itself under load.
>
> When diagnosing "heisenbugs" the key to solving them is isolating the
> moving parts, can you test connectivity to the sql server from some
> client that is not across the serial link. Do packet traces taken on
> the client match those taken on the server?
>
> It does seem perplexing. But it sounds like you have a reasonable
> handle on it.
>
> chris
>
> On Mon, Nov 9, 2009 at 1:32 PM, Jim Ockers <ockers at ockers.net> wrote:
>
>> Hi everyone,
>>
>> It turns out our MySQL server already had the skip-name-resolve option. I
>> ran some packet traces and found the following times without any bandwidth
>> hog and in the presence of a bandwidth hog. The timestamps reported in the
>> server packet trace are within 1-2 milliseconds of the timestamps in the
>> client's packet trace. The delay is the amount of elapsed time from the
>> completion of the TCP 3-way handshake (syn, syn-ack, ack) to when the
>> service banner is received by the client (in my case the "client" is windows
>> telnet.exe):
>>
>> service no-hog start delay hog-start-delay
>> ftp .1ms ~1ms
>> mysql 1ms ~10-15 SECONDS
>> http .1ms ~1ms
>>
>> The Linux server has a mostly idle CPU, load averages around 0.00, and the
>> bandwidth hog is just a FTP download from somewhere else with lots of
>> bandwidth to the Windows client being written to the nul: device. The PPP
>> connection has a maximum of about 750Kbps throughput over the 921600bps
>> RS422 serial link.
>>
>> There is something strange we noticed in the packet trace. The client
>> initiates the TCP 3 way handshake to the server, and then instead of sending
>> the banner the server tries to connect back to the client on port 3306 (it
>> sends a SYN). Of course the client is not running a MySQL server or
>> anything else on port 3306 so it replies with a RST. Then several seconds
>> elapse, then the server finally sends the banner.
>>
>> When the link is not hogged/saturated we don't observe this SYN-RST behavior
>> where the server tries to connect to the client. Google searches about this
>> have not helped. There is nothing in the mysql server config file which
>> would indicate that it should try to connect to its clients on port 3306.
>>
>> When we try this test from a Linux system connected the exact same way we
>> don't get any of this weird behavior with MySQL. However, I don't think it
>> can be OS dependent since the service banner is supposed to be sent right
>> after the TCP 3-way handshake. I don't see how the Linux MySQL server would
>> "know" anything about the client. The 3 way handshake contains no specific
>> information about the client operating system and the packets are small
>> short and to the point. The service banner also seems to be more or less
>> the same regardless of client OS, the difference is timing.
>>
>> When we add an iptables filter/OUTPUT rule preventing the server from
>> connecting to port 3306 on the client's IP address, the hog-start-delay is
>> more like 3 seconds, which is still a long time but is much shorter than
>> without this iptables rule. I think a socket() syscall seems to get a -1 if
>> there is an iptables rule somewhere in the kernel which would prevent that
>> socket from being established. Obviously with this firewall rule in place
>> the SYN does not show up in the packet trace from either end.
>>
>> Also because this whole problem seems to be timing dependent the system does
>> not always behave exactly this way. For example if for some reason there is
>> a TCP retransmission on the bandwidth-hog application which would cause the
>> bandwidth usage to slow down, then the mysql banner at that instant might be
>> faster than it would be had there been no interruption in the bandwidth
>> hogging data flow.
>>
>> This is all too weird, it is giving me a headache. Anyone have any ideas?
>> Sorry for top posting and replying to my own post but I wanted to continue
>> the thread.
>>
>> Thanks,
>> Jim
>>
>> Jim Ockers wrote:
>>
>> Hi Chris,
>>
>> Thank you for the out-of-the-box idea. If MySQL was trying to do inverse
>> DNS it would certainly fail the way you describe because we have not set it
>> up. However I am not sure about that because when the link is not
>> saturated, MySQL responds right away to the same requests or queries. I
>> will try adding a /etc/hosts entry on the linux server end for the client IP
>> address and see if it is any better.
>>
>> Thanks,
>> Jim
>>
>> chris fedde wrote:
>>
>> Having read through this thread I begin to wonder if it is a DNS
>> issue. mysql may be looking for and failing to find reverse dns
>> entries. The characteristic for this particular problem is a 90
>> second delay from the tcp connection till the response banner.
>>
>> You can check to see if this is the issue using tcpdump.
>>
>> On Sun, Nov 8, 2009 at 9:17 PM, Jim Ockers <ockers at ockers.net> wrote:
>>
>>
>> Hi Dave,
>>
>> David L. Anselmi wrote:
>>
>>
>> Jim Ockers wrote:
>>
>>
>> Whenever there is sustained high bandwidth network traffic over the PPP
>> link (such as downloading a large e-mail attachment, or streaming video) the
>> mysql connections all start to time out because response packets from the
>> server are not received at the client. I observed this by doing a simple
>> "telnet server 3306" and observed that the MySQL banner response with
>> version number was delayed by several seconds or until the bandwidth-hogging
>> stopped.
>>
>> What I don't understand is why I can still transfer data from other
>> servers on the Linux system, such as "net view \\server" and "dir
>> \\server\sharename" (using the Windows redirector to talk to Samba on the
>> Linux system). Also the apache web server on the Linux system responds
>> normally, both from a web browser and also in response to "telnet server 80"
>> and then "GET / HTTP/1.0".
>>
>>
>> I assume that other services are delayed like the MySQL banner. Most of
>> what you tried though would use fairly small responses (one packet, even?)
>> so maybe the MySQL protocol is more verbose.
>>
>>
>>
>> No, actually the only service that is delayed is MySQL response packets
>> including the banner. Apache responds right away to a GET / HTTP/1.0
>> request, samba responds right away to a Windows redirector request, FTP
>> responds right away, and so forth. MySQL is the only one that seems to take
>> an unreasonably long time to respond when there is something hogging the
>> bandwidth.
>>
>>
>> It seems that clients have several timeouts available:
>>
>> http://dev.mysql.com/doc/refman/5.1/en/mysql-options.html
>>
>> Maybe the protocol takes too long in some cases.
>>
>>
>> I'm not too sure about that, but since the banner is delayed I wonder if
>> MySQL has some sort of built-in congestion management or something.
>>
>> Thanks though,
>> Jim
>>
>>
>>
>>
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