bought 2 pieces of Crucial 8G RAM, together with the stock will be 4+4+8+8=24G of RAM.
when i opened up the RAM cover, it was stock1, empty1, stock2, empty2 (4,-,4,-).
installed them in the format of: stock1, stock2, new1, new2(4,4,8,8); even the Apple Support site didn't say anything about format or configuration!
after this, i could not power on the iMac... 😕😮😟
so quickly try to change them to stock1, new1, stock2, new2. (4,8,4,8) the iMac can power up. 😉
Lesson learnt: just install the new ones into the empty slots.
So there is actually so kind of configuration that we needed to follow? But Apple Support site did not mention any of these.
Now the RAM is upgraded from 8G to 24G.
However, i did a test to process a batch of 300 photos in Lightroom, it only improve timing from 9min 25sec to 8min 41sec, just a little bit faster, not impressive!