Lighttpd and the wonders of strace

I ran Lighttpd under strace today whilst debugging a problem with mod_deflate and I found two mis-configurations just from watching the system calls it was making. In case anyone is interested, this is what I found.

Firstly, I’d enabled the system.use-noatime option but I could see that it was failing to set this mode when opening a file to serve:

open("/home/john/.../newsniffer.css", O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_NOATIME) = -1 EPERM (Operation not permitted)
open("/home/john/.../newsniffer.css", O_RDONLY|O_NONBLOCK|O_LARGEFILE) = 40

I realised that Lighty drops privileges on start-up, and the O_NOATIME option is privileged. No biggie, but a wasted system call is a wasted system call, so I disabled the option. To my surprise, this fixed the blank/empty page problem I was having with mod_deflate. Clearly a bug, but now I can file a slightly more helpful bug report (this is the Lighttpd dev trunk btw).

Secondly, I’d always assumed (having read it somewhere, I’m sure) that Lighttpd selected the most efficient event-handler available on the operating system – on my Linux 2.6 system this would be epoll, but strace showed Lighttp using regular poll:

poll([{fd=4, events=POLLIN}, {fd=5, events=POLLIN}, {fd=-1}, {fd=-1}, {fd=-1}, {fd=-1}], 6, 1000) = 0

So, I explicitly configured it with server.event-handler = "linux-sysepoll" and now strace shows me:

epoll_wait(39, {}, 4096, 1000)          = 0

I’ve always found the strace tool very useful, but sometimes I forget and take it for granted. I love you strace tool.


andrew says:

Right said, John. strace saved so many of my days too. Tools like that really make the difference and one should never forget.

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