The program blocks, but NOT when it goes through strace

Question

The program blocks, but NOT when it goes through strace

I am doing server side rendering with wkhtmltoimage and every launch is blocked at "88% load" for 1-2 minutes. I decided to debug what was happening through strace, but in a completely strange twist, the program did NOT lock. I found this completely repeatable and consistent. Why would the fear of making the program faster, when by all rights the program should be slower ?!

Run without strace:

user@server:~/public_html/shapes$ time wkhtmltoimage --disable-smart-width --width 970 --format jpg '[THE URL]' '[THE PATH].jpg'
Loading page (1/2)
Rendering (2/2)
Done

real    1m45.724s
user    1m42.887s
sys     0m0.623s

Running with strace:

user@server:~/public_html/shapes$ time strace wkhtmltoimage --disable-smart-width --width 970 --format jpg '[THE URL]' '[THE PATH].jpg'
execve("/usr/local/bin/wkhtmltoimage", ["wkhtmltoimage", "--disable-smart-width", "--width", "970", "--format", "jpg", "[THE URL]"..., "[THE PATH]"...], [/* 21 vars */]) = 0
brk(0)                                  = 0x311a000
...
exit_group(0)                           = ?
+++ exited with 0 +++

real    0m6.526s
user    0m0.582s
sys     0m0.377s

The server is closed, so I edited the URL and PATH, but otherwise the output will be correct. Also both runs produce correct output and I have cleaned up the temp files to ensure this is not a caching issue. I did these runs 10 times each to ensure that this is not a random artifact, but it happens in sequence. SO the only logical conclusion is that strace somehow changes the behavior of wkhtmltoimage, and I was really hoping someone would tell me. If I knew why strace is forcing the program not to block, I could find a solution.

Here's the hanging process:

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
  772 sigapp    20   0 1485600  45760  14524 R  99.8  2.2   0:57.02 wkhtmltoimage

As root, I can attach to the dangling image with strace -p $(pidof wkhtmltoimage)

, and the result is:

gettimeofday({1436692542, 446246}, NULL) = 0
gettimeofday({1436692542, 556958}, NULL) = 0
gettimeofday({1436692542, 557161}, NULL) = 0
gettimeofday({1436692542, 659238}, NULL) = 0
gettimeofday({1436692542, 771381}, NULL) = 0
gettimeofday({1436692542, 771686}, NULL) = 0
gettimeofday({1436692542, 875783}, NULL) = 0
gettimeofday({1436692542, 987490}, NULL) = 0
gettimeofday({1436692542, 987781}, NULL) = 0
gettimeofday({1436692543, 84764}, NULL) = 0
...

System: Linux Server 3.13.0-30-generiC # 55-Ubuntu SMP Fri Jul 4 21:40:53 UTC 2014 x86_64 x86_64 x86_64 GNU / Linux (runs on 2 Xen guest machines)

Software: wkhtmltoimage 0.12.2.1 (with fixed qt) <- installed from official site via .deb file

I read this but not sure if this is relevant: Hoon processes are resumed if they are attached to strace

UPDATE:

Tried with another program webkit2pdf

and also saw locks. This time the strace output is different. Both programs use webkit .:

root@server:~# strace -p `pidof webkit2pdf`
Process 4699 attached
restart_syscall(<... resuming interrupted call ...>

On successful launch, strace wkhtmltoimage ...

I can see similar calls, but not as often as many preceded mmap:

mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x7f9dbbbc1000
gettimeofday({1436696839, 361942}, NULL) = 0
gettimeofday({1436696839, 362254}, NULL) = 0
gettimeofday({1436696839, 362505}, NULL) = 0
gettimeofday({1436696839, 362787}, NULL) = 0
gettimeofday({1436696839, 363172}, NULL) = 0
gettimeofday({1436696839, 363568}, NULL) = 0
gettimeofday({1436696839, 363913}, NULL) = 0
mmap(NULL, 28672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x7f9db5000000
gettimeofday({1436696839, 364701}, NULL) = 0
mmap(NULL, 28672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x7f9db4ff9000
gettimeofday({1436696839, 365612}, NULL) = 0
gettimeofday({1436696839, 365956}, NULL) = 0