nginx not responding to some SYN packets

[Igor Sysoev]

On Tue, Feb 03, 2009 at 10:07:27PM -0800, Radek Burkat wrote:

> Have a machine running the latest devel version nginx-0.7.33 (tried 0.6.35
> with same results) for serving small (less than 10K images) and am seeing on
> tcpdump that some SYN packets are not responded to right away.The browser
> does retransmit these image requests every second and on the 2nd or 3rd
> resent SYN, I finally start seeing and ACK, and the images load.
> It is very indeterministic as to when it happens and can only reproduce it
> some of the time.  When it does occur the outcome is a page with some images
> loaded and others (whose SYN packets are not ACKs) are not loaded.....a few
> seconds later they load.
> Typically the system has ~2000 active connections, most in keep alive.  The
> load is around 100-200 req/sec.
> 
> I have tries all sorts of settings and configurations suggested in the
> maillist but I still dont have the solution for this issue.  from 1 to 4
> workers, changing the connection counts, different even handlers, kernel
> buffers, etc.
> It just seems so anecdotal to just change a bunch of settings without being
> able to what is happening internally.
> I'd like to be able to debug a little deeper to find out what is happening
> to these packets.
> 
> How would I go about debugging what is the cause of it.  Is it the interface
> driver, kernel, or nginx?  What kind of tools and debugging options can I
> try next?

nginx has not any relation to TCP handshake (SYN, SYN/ACK, ACK),
this is kernel or driver issue.

> Thanks, Radek
> 
> 
> System Details
> model name : Intel(R) Xeon(R) CPU X3210 @ 2.13GHz
> Linux n06 2.6.18-92.1.22.el5 #1 SMP Tue Dec 16 12:03:43 EST 2008 i686 i686
> i386 GNU/Linux
> eth0
> Advertised auto-negotiation: Yes
> Speed: 1000Mb/s
> Duplex: Full
> Port: Twisted Pair
> PHYAD: 1
> Transceiver: internal
> Auto-negotiation: on
> Supports Wake-on: g
> Wake-on: d
> Current message level: 0x000000ff (255)
> Link detected: yes
> driver: tg3
> version: 3.86
> firmware-version: 5721-v3.61, ASFIPMI v6.21
> bus-info: 0000:05:00.0
> 
> avg-cpu: %user %nice %system %iowait %steal %idle
> 0.10 0.00 0.20 2.43 0.00 97.27
> Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm
> %util
> sda 0.00 0.00 27.40 0.00 443.20 0.00 16.18 0.10 3.50 3.32 9.10
> no overruns or errors on interface.

-- 
Igor Sysoev
http://sysoev.ru/en/