Nginx stops sending file after ~1.5mb ?

Igor Sysoev is at rambler-co.ru
Tue Jan 27 12:00:33 MSK 2009 

On Mon, Jan 26, 2009 at 02:05:28PM +0200, Yo'av Moshe wrote:

> Hey, attached is my tcpdump log.
> I've never used tcpdump before, so I hope the log is fine.
> I used grep to show only the connections from my computer while running
> 'wget http://www.noal.org.il/static/temp/hozer.pdf'.
> 
> Hope this will lead us somewhere...

In this tcpdump I see the only strange thing: client acknowledged
data that was ~64K before (1534561), and this may be usual thing,
then server started to retransmit data and client never acknowledged
the data (1535941).

13:53:53.739626 49926 > http: . 125:125(0) ack 1534561 win 65535
13:53:53.739679 http > 49926: . 1596661:1598041(1380) ack 125 win 5840
13:53:53.739865 http > 49926: . 1598041:1599421(1380) ack 125 win 5840
13:53:54.577857 http > 49926: . 1534561:1535941(1380) ack 125 win 5840
13:53:56.256478 http > 49926: . 1534561:1535941(1380) ack 125 win 5840

However, when I have tried to look tcpdump on my side, I saw the my
client sent all acknowledges, but did not get a new data on some point.

> Yo'av.
> 
> On Sun, Jan 25, 2009 at 9:21 PM, Igor Sysoev <is at rambler-co.ru> wrote:
> 
> > On Sun, Jan 25, 2009 at 09:04:54PM +0200, Yo'av Moshe wrote:
> >
> > > Sendfile is now off, using select, and still the same...
> > > http://pastebin.com/m2c3ea25b
> >
> > I have no idea, why it may be so.
> >
> > I see that select() returned event ("out [19]"):
> >
> > 28215 20:50:18 select(20, [10 19], [19], NULL, {0, 500000} <unfinished ...>
> > 28215 20:50:18 <... select resumed> )   = 1 (out [19], left {0, 380000})
> >
> > However, at some stage select() did not return event, although the event
> > is still active ("select(20, [10 19], [19], NULL ..."):
> >
> > 28215 20:50:18 gettimeofday({1232909418, 293895}, NULL) = 0
> > 28215 20:50:18 writev(19,
> > [{"\227\1\2\1\3\1\4\1\5\0\373\0\374\1\230\1\231\1\232\1\233\0\375\0\376\1\6\1\7\1\10\0"...,
> > 9676}], 1) = 9676
> > 28215 20:50:18 pread64(20,
> > "\24\7\6#\"\'&\0214\22$63\1\6\25\24\26\27\02632654&#\"\6\3\226\204"...,
> > 32768, 1605632) = 32768
> > 28215 20:50:18 writev(19,
> > [{"\24\7\6#\"\'&\0214\22$63\1\6\25\24\26\27\02632654&#\"\6\3\226\204"...,
> > 32768}], 1) = 32768
> > 28215 20:50:18 pread64(20, "\0 \0M\0o\0r\0i\0s\0o\0n\0\'\0s\0
> > \0d\0i\0r\0e\0c"..., 32768, 1638400) = 32768
> > 28215 20:50:18 writev(19, [{"\0 \0M\0o\0r\0i\0s\0o\0n\0\'\0s\0
> > \0d\0i\0r\0e\0c"..., 32768}], 1) = 1716
> > 28215 20:50:18 select(20, [10 19], [19], NULL, {0, 500000} <unfinished ...>
> > 28215 20:50:18 <... select resumed> )   = 0 (Timeout)
> >
> > Could you run tcpdump of the session ?
> >
> > > Yo'av
> > >
> > > On Sun, Jan 25, 2009 at 7:33 PM, Igor Sysoev <is at rambler-co.ru> wrote:
> > >
> > > > On Sun, Jan 25, 2009 at 06:26:58PM +0200, Yo'av Moshe wrote:
> > > >
> > > > > Igor, I tried using select instead of poll, but I get the exact same
> > > > > problem.
> > > > >
> > > > > Check my strace:
> > > > > http://pastebin.com/m65056ec0
> > > > >
> > > > > I get the same thing with rtsig too.
> > > > >
> > > > > Hints?
> > > > > Again, file is at http://www.noal.org.il/static/temp/hozer.pdf.
> > Server
> > > > is
> > > > > using select now.
> > > >
> > > > According strace select() does not return event for socket 19 after
> > > > sendfile64() sent 1651005 bytes:
> > > >
> > > > 25951 18:14:26 select(20, [7 8 16 19], [19], NULL, {60, 0} <unfinished
> > ...>
> > > > 25951 18:14:27 <... select resumed> )   = 1 (out [19], left {58,
> > 790000})
> > > >
> > > > 25951 18:14:27 gettimeofday({1232900067, 890376}, NULL) = 0
> > > > 25951 18:14:27 sendfile64(19, 20, [1509465], 440765 <unfinished ...>
> > > > 25951 18:14:27 <... sendfile64 resumed> ) = 141540
> > > > 25951 18:14:27 sendfile64(19, 20, [1651005], 299225 <unfinished ...>
> > > > 25951 18:14:27 <... sendfile64 resumed> ) = -1 EAGAIN (Resource
> > temporarily
> > > > unavailable)
> > > >
> > > > 25951 18:14:27 select(20, [16 19], [19], NULL, {0, 500000} <unfinished
> > ...>
> > > > 25951 18:14:28 <... select resumed> )   = 0 (Timeout)
> > > >
> > > > As you have got the same issue with all methods (even with level
> > triggered
> > > > select()), I believe that the bug is in sendfile. You may try to turn
> > > > it off even for the single file:
> > > >
> > > >    location = /static/temp/hozer.pdf {
> > > >        sendfile off;
> > > >    }
> > > >
> > > > In Linux 2.6.23 sendfile() has been rewritten to use splice framework.
> > > > The bug may be introduced while rewriting.
> > > >
> > > > BTW, it seems that thttpd does not use sendfile.
> > > >
> > > > > Thanks...
> > > > > Yo'av.
> > > > >
> > > > > On Fri, Jan 23, 2009 at 3:05 PM, Igor Sysoev <is at rambler-co.ru>
> > wrote:
> > > > >
> > > > > > On Fri, Jan 23, 2009 at 02:35:00PM +0200, Yo'av Moshe wrote:
> > > > > >
> > > > > > > Thanks Igor.
> > > > > > > I don't want to be rude, but I find it hard to believe that it's
> > a
> > > > kernel
> > > > > > > bug.
> > > > > > >
> > > > > > > First, I'm using the default CentOS 5.2 kernel, it's seems to me
> > > > pretty
> > > > > > odd
> > > > > > > that there's such a bug in it. CentOS is being used on a lot of
> > > > servers,
> > > > > > > some of them must be running nginx.
> > > > > > >
> > > > > > > Second, I tried serving the same file with thttpd, and it runs
> > just
> > > > fine.
> > > > > > > http://195.28.180.147:40/temp/hozer.pdf
> > > > > > > Note it's the exact same file which I cannot get from nginx:
> > > > > > > http://www.noal.org.il/static/temp/hozer.pdf
> > > > > > >
> > > > > > > How come thttpd can serve the file just fine? If it's a kernel
> > bug, I
> > > > > > > thought it would happen with all of my servers.
> > > > > > > If it's using some different way of sending the files which does
> > not
> > > > > > depend
> > > > > > > on that epoll call - is there any way to configure nginx to use
> > that
> > > > as
> > > > > > > well?
> > > > > >
> > > > > > I do not look modern thttpd, but according its change log, it has
> > no
> > > > > > epoll support. You may try different ways in nginx: you need to
> > build
> > > > it
> > > > > >
> > > > > >        --with-rtsig_module
> > > > > >        --with-select_module
> > > > > >        --with-poll_module
> > > > > >
> > > > > > and then choose method:
> > > > > >
> > > > > > events {
> > > > > >     use  select;
> > > > > >     #use  poll;
> > > > > >     #use  rtsig;
> > > > > > }
> > > > > >
> > > > > > Also, epoll has two modes: level triggered (default) and edge
> > > > > > triggered (EPOLLET). nginx uses more effective edge triggered mode,
> > > > > > lighttpd uses level triggered one. The bug may be in ET mode only.
> > > > > >
> > > > > > As to probabilty of kernel bugs: I saw them in FreeBSD (kqueue,
> > > > sendfile),
> > > > > > Linux (epoll), Solaris (event ports), and MacOSX (kqueue,
> > sendfile).
> > > > > >
> > > > > > > Can I do anything to confirm that it's a kernel bug? If it is, I
> > > > should
> > > > > > > probably contact CentOS's maintainers.
> > > > > >
> > > > > > Your straces confirms this: nginx added socket to epoll and did not
> > > > > > deleted it, nevertheless epoll does not send event.
> > > > > >
> > > > > > > Thank you, again.
> > > > > > > Yo'av.
> > > > > > >
> > > > > > > On Wed, Jan 21, 2009 at 7:26 AM, Igor Sysoev <is at rambler-co.ru>
> > > > wrote:
> > > > > > >
> > > > > > > > On Wed, Jan 21, 2009 at 01:52:53AM +0200, Yo'av Moshe wrote:
> > > > > > > >
> > > > > > > > > Igor, here's the new strace.
> > > > > > > > > http://pastebin.com/m5129c54b
> > > > > > > > >
> > > > > > > > > Does the fact it works in low download rates still makes you
> > > > think
> > > > > > it's a
> > > > > > > > > kernel issue?
> > > > > > > >
> > > > > > > > Yes, this is seems like a kernel issue. Kernel excatly said
> > (with
> > > > > > EAGAIN)
> > > > > > > > that there is no place to output data, but never returned a new
> > > > > > > > notification
> > > > > > > > via epoll_wait() about free space:
> > > > > > > >
> > > > > > > > 21420 01:39:43 <... epoll_wait resumed> {{EPOLLOUT,
> > {u32=153137960,
> > > > > > > > u64=13811628390360331048}}}, 512, 60000) = 1
> > > > > > > > 21420 01:39:43 gettimeofday({1232494783, 413764}, NULL) = 0
> > > > > > > > 21420 01:39:43 sendfile64(20, 21, [1645625], 304605) = 40320
> > > > > > > > 21420 01:39:43 sendfile64(20, 21, [1685945], 264285) = -1
> > EAGAIN
> > > > > > (Resource
> > > > > > > > temporarily unavailable)
> > > > > > > > 21420 01:39:43 epoll_wait(11,  <unfinished ...>
> > > > > > > >
> > > > > > > > This is probably some race condition inside kernel, that does
> > not
> > > > > > appear on
> > > > > > > > low speed download.
> > > > > > > >
> > > > > > > > > Thank you, really!
> > > > > > > > > And thanks, Kiril and Jeff, for confirming that!
> > > > > > > > >
> > > > > > > > > Yo'av.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > On Tue, Jan 20, 2009 at 9:39 PM, Kiril Angov <
> > > > kupokomapa at gmail.com>
> > > > > > > > wrote:
> > > > > > > > >
> > > > > > > > > > wget http://www.noal.org.il/static/temp/barvazi2.pdf
> > > > > > > > > > --2009-01-20 21:36:00--
> > > > > > > > http://www.noal.org.il/static/temp/barvazi2.pdf
> > > > > > > > > > Resolving www.noal.org.il... 195.28.180.147
> > > > > > > > > > Connecting to www.noal.org.il|195.28.180.147|:80...
> > connected.
> > > > > > > > > > HTTP request sent, awaiting response... 200 OK
> > > > > > > > > > Length: 1950230 (1.9M) [application/pdf]
> > > > > > > > > > Saving to: `barvazi2.pdf'
> > > > > > > > > >
> > > > > > > > > > 81% [=====================================>         ]
> > 1,596,665
> > > > > > > > > > --.-K/s  eta 5s
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > wget -c http://www.noal.org.il/static/temp/barvazi2.pdf
> > > > > > > > > > --2009-01-20 21:38:28--
> > > > > > > > > > http://www.noal.org.il/static/temp/barvazi2.pdf
> > > > > > > > > > Resolving www.noal.org.il... 195.28.180.147
> > > > > > > > > > Connecting to www.noal.org.il|195.28.180.147|:80...
> > connected.
> > > > > > > > > > HTTP request sent, awaiting response... 206 Partial Content
> > > > > > > > > > Length: 1950230 (1.9M), 305960 (299K) remaining
> > > > [application/pdf]
> > > > > > > > > > Saving to: `barvazi2.pdf'
> > > > > > > > > >
> > > > > > > > > > 100%[+++++++++++++++++++++++++++++++++++++++=======>]
> > 1,950,230
> > > > > > > > > > 295K/s   in 1.0s
> > > > > > > > > >
> > > > > > > > > > 2009-01-20 21:38:30 (295 KB/s) - `barvazi2.pdf' saved
> > > > > > [1950230/1950230]
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > On Tue, Jan 20, 2009 at 9:31 PM, jeff emminger <
> > > > > > jemminger at gmail.com>
> > > > > > > > > > wrote:
> > > > > > > > > > >> Could anyone confirm that this works:
> > > > > > > > > > >> wget --limit-rate=3500
> > > > > > > > http://www.noal.org.il/static/temp/barvazi2.pdf
> > > > > > > > > > >> while this doesn't:
> > > > > > > > > > >> wget http://www.noal.org.il/static/temp/barvazi2.pdf
> > > > > > > > > > >>
> > > > > > > > > > >
> > > > > > > > > > > Confirmed
> > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > --
> > > > > > > > > Yo'av Moshe
> > > > > > > >
> > > > > > > > --
> > > > > > > > Igor Sysoev
> > > > > > > > http://sysoev.ru/en/
> > > > > > > >
> > > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > --
> > > > > > > Yo'av Moshe
> > > > > >
> > > > > > --
> > > > > > Igor Sysoev
> > > > > > http://sysoev.ru/en/
> > > > > >
> > > > > >
> > > > >
> > > > >
> > > > > --
> > > > > Yo'av Moshe
> > > >
> > > > --
> > > > Igor Sysoev
> > > > http://sysoev.ru/en/
> > > >
> > > >
> > >
> > >
> > > --
> > > Yo'av Moshe
> >
> > --
> > Igor Sysoev
> > http://sysoev.ru/en/
> >
> >
> 
> 
> -- 
> Yo'av Moshe



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

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