Photoshop Elements Memory Question

Win XP Pro SP3 4GB Physical Memory - Photoshop Elements v4:

Task manager shows available physical memory as 2.6GB. Photoshop preferences shows 1.744GB available memory. Why the difference?

Another computer running same OS with 2GB physical memory:

Task manager shows 1.5GB available physical memory. Photoshop preference shows 1.755GB available which is actually more than really exists. Why the difference?

On both computers Photoshop reports the wrong amount of available memory - one computer too much - other computer not enough. I am trying to understand how this works. Can someone help?

Windows normally makes 2GB of address space available to an application - in other words an application believes it can allocate up to 2GB of memory, less the space it's already occupying, and Windows will provide it. It's up to the Windows Memory Manager to determine whether this 2GB quantity can be allocated from available RAM (physical memory) or from hard drive space appearing to be memory (virtual memory in the paging file). This management by Windows is unknown to the program using the memory.

So Photoshop reports how much of that available 2GB is not currently being used for any function by the program, hence is still available. But it doesn't know anything about where that memory is coming from.

Behind the scenes, Windows makes the amount up in a different way on the two computers, and because Windows does know how its memory complement is being put together, it can show you different figures to what Photoshop knows about.

If that is the case, then why does Photoshop preference setting, by default, assign only about 55% of that RAM which has already been assigned to it?

It's a balancing act between starting out with too little or too much memory pre-allocated, either of which can affect Photoshop's performance.

It's a big program that will be handling big images. At startup, memory structures for file handling, cacheing and processing are initialised. Photoshop claims this memory exclusively for its own use, both for instantaneous access, and to have contiguous memory (not fragmented).

If it starts out allocating a high proportion of the memory available, then even if the system has 4GB of RAM fitted, it's likely that Windows' or Photoshop's own operations are going to be paged out to virtual memory when Windows runs out of RAM, a big hit on performance. There also has to be some memory space left available for other things that could need to start up and run in the same space as Photoshop, such as plug-ins.

Assigning a lower initial figure allows for more memory to be dynamically allocated if it's needed, then released back to the memory pool when no longer needed, reducing the likelihood of Windows running out of RAM and having to use the paging file on the hard drive.

If a system has 4GB of RAM fitted, you can experiment with increasing the Preference allocation up to about 70%, reducing it if you find you hit page file activity too readily.

Thanks for the information. You have given me a good education. I appreciate it.

You're welcome. I also pick up extra bits of knowledge by researching questions folk ask.