I did some more research on this problem:
First I started to listen Spotify.
Test 1: I pooled cpufreq readings (while() do cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq; sleep 15; done) and waited about 5 minutes. Result: 200Mhz, almost constant. Then I turned the screen OFF and waited for 10 minutes until the phone began to feel hot. I checked cpufreq pools. Result 1200Mhz, constant, at least 3-4 minutes. Cpufreq returned to normal after I turned the screen ON.
Test 2: top -m 20 -d 60. I waited 5 minutes. Result: spotify cpu usage 4-17%. Then I turned the screen OFF and waited for 15 minutes until the phone began to feel hot, again. I checked the latest top output. Result: spotify cpu usage 49%. Cpu usage returned to normal after I turned the screen ON.
If I understand this correctly, this means that the spotify retains a whole cpu core for itself forcing it to run at full clock. This might be the real reason why spotify kills my battery.
I reported this to the spotify-group by email. I hope that they will do something about this problem.
http://getsatisfaction.com/spotify/topics/gui_process_uses_a_whole_cpu_core_when_screen_is_off
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UPDATE:
I just received a reply to the original e-mail: "
Please note that the Galaxy SII is not supported by Spotify yet. Having a Galaxy SII (dual core) app is something we want to do at some point but as with all development plans we don't discuss future releases. When we have something to say about a dedicated dual core app we'll be sure to let everyone know."
I suspect this has nothing to do with dual core processor. My understanding is that Android 2.3.* doesn't even have a full dual core -support (meaning that normal app sees only one core, OS two). Correct me if I am wrong... I would guess (based on my Wildfire+spotify experience) that this problem occurs with other phones too. SGS2s more powerful processor just makes things worse. Both cores run at 1200Mhz and one at full speed...
I am testing review-version (0.4.9.175). Preliminary results seem good. No heating yet.
I posting future updates into: http://getsatisfaction.com/spotify/topics/gui_process_uses_a_whole_cpu_core_when_screen_is_off
and
http://androidforums.com/android-applications/359753-spotify-makes-phone-hot-kills-battery.html