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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?

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Baumr
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    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. – Nicholas Smith 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
  • I don't think the newer Macbook actually have more than an 8Gb limit at all, just the iMacs and up. It sounds like if it's pegging the fans it's really trying to suck down the cache and struggling. – Nicholas Smith Mar 21 '13 at 15:07
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    @NicholasSmith, yup new Mac's have 16GB. – Baumr Mar 21 '13 at 15:23
  • Ah hah! Sorry, I never look over 13". At a guess I'd assume it's been tweaked in a specific customised build (which would be weird) or more likely somewhere in firmware. However, I can't find anymore reports like yours, so my idea might be out. – Nicholas Smith Mar 21 '13 at 15:26
  • @NicholasSmith, I should have mentioned mine is the 15" earlier — edited my question! Thanks – Baumr Mar 21 '13 at 15:35
  • It seems odd you wouldn't swap back in the proper RAM or at least pull one of the chips and test. Perhaps you are going to do that and then answer your question jeopardy style? – bmike Mar 21 '13 at 15:55
  • @bmike, what do you mean? The 4GB RAM that came with it doesn't exhibit that behavior. So should I take one chip out, and put in an old one, to test again? – Baumr Mar 21 '13 at 16:22
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    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
  • Let me take a wild guess. The fan was not doing that before ? –  Mar 21 '13 at 17:50
  • there is nothing wrong with fan spinning faster for a while. –  Mar 21 '13 at 21:28
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    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. – Graham Perrin Mar 25 '13 at 19:27

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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.

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    Please add a link to a knowledge base article or similar content. – Dave Nelson Mar 21 '13 at 17:07
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    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
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    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. –  Mar 21 '13 at 17:41
  • You answered your question, yes sleep mode is more power demanding then shut down. –  Mar 21 '13 at 22:00