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Currently, if I open Photoshop after a days time, the load up time is about 5 seconds. If I close and open again (right after), the load up time is about 2 seconds (I assume this is caching?). Can someone explain how adding memory will (if at all) increase performance here (4GB to 16GB)?

Background Info: My Page-Ins are about 2GB and my Page-Outs are about 60MB (iMac 12,2 4GB RAM).

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You'll know you have enough RAM when starting Photoshop doesn't page out much else to disk and instead fills unallocated RAM. –  bmike Aug 16 '11 at 20:23
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3 Answers

The increase speed you see when starting Photoshop immediately after previously starting and stopping it is coming from the hard disk cache which cannot be increased, and adding more system memory will not make much difference unless your allocated memory is sufficiently high to require significant paging out to make room for the new app. As any paging will have taken place on the first load, it's unlikely to require more paging on the 2nd attempt unless you re-access the data that has been paged out so that is get's paged back in, which you do not allow time for in this scenario. Your page in/out figures need to be better detailed to allow for proper consideration - how long is this sampled over etc?

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Odd - my macs with 4 gb ram read far more data than can fit in the drive cache starting CS4 and CS5 both first and subsequent launches. I get similar timings as the OP on iMacs with fast internal drives. 8g ram is the sweet spot for me on this use case. –  bmike Aug 16 '11 at 20:09
    
Reading back while I still think what I said was accurate, the effects are probably less marked with a huge app like CS4, so drive cache is probably the lesser effect here, and the fact that 2 loads in a row pages out everything that isn't required without a chance for it to get paged back in makes for the biggest difference. –  stuffe Aug 16 '11 at 20:14
    
My rule of thumb is most macs will empty a 32 mb cache in a third of a second normal IO most of the time. it really rocks small files like Xcode. Most drives cache the writes more than the reads afaik. –  bmike Aug 16 '11 at 20:21
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If your system is stable enough to not need a reboot between launches, then you might have a drastic improvement by jumping that much RAM.

It's more about what else causes things to page out in the mean time than how photoshop behaves. In practice with Lion and Snow Leopard - it seems that the difference between 4 and 8 G of RAM makes a noticeable speedup after several days use have the programs you use resident in RAM. Going to 16 would be heaven to me. Even 5 seconds is pretty fast, so don't get it if you are going to be disappointed if it's 4 or 5 after the upgrade.

Other things will sped up for sure, but it's hard to know if your workload will guarantee 2 seconds with any amount of RAM if the inactive pages that hold Photoshop eventually get released by the virtual memory system to make room for newer inactive pages.

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If you have 60MB of page-outs over any significant uptime, then your machine is not memory-constrained and you won't see a significant improvement by adding memory. With next to no page-outs, your page-in represents programs loaded into memory when you run them. You aren't going to make that happen any faster unless you buy an SSD.

If after day of running a normal workload you have lots of page-outs, then you might benefit from adding memory. n.b., looking at free memory in Activity Monitor only shows a piece of the puzzle, as Unix will tend to keep things in memory once they are loaded.

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