I recently:

  • installed 16 GB of RAM (1600 MHz);
  • on my late-2011 15-inch MacBook Pro;
  • running OS X 10.7, aka Lion;
  • with an Apple SSD drive.

According to Apple my MacBook Pro only supports a maximum of 8 GB at 1333 MHz, but the chipset supports more (both GB and MHz) — but I'm aware that it does not mean it'll actually be compatible with my machine.

The problem:

Whenever I put the laptop to sleep (by closing it), it takes a while, and the fans start spinning very loudly before the computer finally reaches sleep mode.

The reason is likely to be Safe Sleep — but why are the fans going crazy?

Is there something wrong with my Mac?

How do I prevent this without disabling Safe Sleep?

Would updating to OS X 10.8, Mountain Lion, help?

  • 2
    I won't leave this as answer as it's more a wild punt, but when you put the Mac to sleep it does essentially dump the contents of the RAM into the sleep file. I think the 8Gb cap might be there for that reason. Mar 21 '13 at 14:57
  • @NicholasSmith, interesting thought, so is the cap increased for newer models through the the OS software or firmware?
    – Baumr
    Mar 21 '13 at 15:00
  • 1
    @NicholasSmith, yup new Mac's have 16GB.
    – Baumr
    Mar 21 '13 at 15:23
  • 3
    Between this comment and @borderline 's answer, it might just be that writing the sleep image at 4 GB isn't noticeable, but when your mac needs to write 16 GB of sleep image, it takes 4 times as long before the air vent is done.
    – bmike
    Mar 21 '13 at 17:08
  • 1
    If the battery is suitably charged, then safe sleep is not the reason. See the first situation at Mac computers: Progress bar appears after waking from sleep. Do not imagine that 16 GB is written to disk every time you simply sleep the Mac. Mar 25 '13 at 19:27

That is normal function. It is called air vent (power air purge), to remove the residual heat before shutting down (or sleep mode).

It is meant to prolong the life of your devices.

The designers have figured out that by shutting a hot computer down, since there is no ventilation the heat actually rises inside. That is not so good for the computer so someone had the bright idea to do the air purge before shutting down. :)

However! one should say that you are running your computer at the "Borderline" (hehe) of capacity for cooling since you stuffed it up with more heating sources.

  • 1
    Please add a link to a knowledge base article or similar content. Mar 21 '13 at 17:07
  • 2
    This might explain things if the air vent waits for the write of all RAM to storage before sleeping. The time to write 16 GB of RAM might easily be 4 or more times as long as it takes to write a smaller RAM image to drive for Safe Sleep / Hibernate Mode.
    – bmike
    Mar 21 '13 at 17:10
  • 4
    Cooling fans used to be stupid. Now they are very intelligent to prolong the computer life. First there is a higher activity during shut down so more cooling is required. Second since it knows it will be shut down and there will be a residual heat, it spins up. On some models from Sony, the fan keeps running even if the computer is shut down (black screen). There is a latency between the computer operation and heat rising, so the fan actually does not wait, but reacts to the temperature sensor which is much slower than the computer operations.
    – user44516
    Mar 21 '13 at 17:41
  • You answered your question, yes sleep mode is more power demanding then shut down.
    – user44516
    Mar 21 '13 at 22:00

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