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#1 ishkabible  Icon User is offline

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Ram, Ramdisks, 32 and 64-bit, and different OSs

Posted 18 June 2013 - 04:38 PM

So I'm going to build a new system pretty soon. I know, by and large, what I am going to build at this point but I have a few remaining questions regarding mem-swapping.

here goes:
  • If I have a 64-bit operating system and greater than 4 Gb of ram can the OS have 32-bit virtual memory paged in areas of physical memory higher than 64-bit?
  • If not It seems logical that I could do this with a Ramdisk by having mem-swapping occur on a hard-drive that is basically as fast as RAM. This would give the ability to run more 32-bit applications effectively in memory at once. Would this even be recommended or is there some issue with this?
  • I already know that I will be using a 256 Gb SSD, I won it in a programming competition. If I have a RAMDisk setup is it possible to mem-swap to both
  • If so is it possible to do in such a way that more frequently accessed pages will be on the RAMDisk rather than the SSD
  • If I do have a setup where mem-swaping can be distributed and prioritized across a RAMDisk and a SSD what kind of performance hit would a single 64-bit application take by having to mem-swap to the RAMDisk sometimes?
  • For anytime I asked "is it possible" would this only be possible on certain OS or are these things widely supported?


if it helps these are the following things this rig will be used for:
*some quasi-intensive numerical computing. I work for a machine learning research group that sometimes has to do some numerical computing that a) takes a lot of memory and B) is easy to make parallel(I plan to use CUDA for this)
*gaming. this wont really use all that much RAM but it will be used for this quite a bit
*For sure it will be used as a private file server for me.
*There is a good chance it will be used host some web services over a local network which will be spawning instances of Java. The load could be as high as 20 people(very unlikely however, 2-3 would be an average case) doing some kind of computing involving the processing of potentially large amounts of text. This would however be temporary and frankly shouldn't be much of a deciding factor.
*If the hardware is there to do it, I will most likely make use of it in programming just playing around with things(I really want to look at sorting as big an array as I can on my new and old hardware :P to see the diffrence)

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Replies To: Ram, Ramdisks, 32 and 64-bit, and different OSs

#2 jjl  Icon User is offline

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Re: Ram, Ramdisks, 32 and 64-bit, and different OSs

Posted 24 June 2013 - 03:38 PM

Quote

If I have a 64-bit operating system and greater than 4 Gb of ram can the OS have 32-bit virtual memory paged in areas of physical memory higher than 64-bit?


I believe a 32-bit virtual memory address can address any portion of the memory, since the virtual memory address is simply a page number plus a offset. What do you mean by "areas of physical memory higher than 64-bit"?

Quote

If not It seems logical that I could do this with a Ramdisk by having mem-swapping occur on a hard-drive that is basically as fast as RAM. This would give the ability to run more 32-bit applications effectively in memory at once. Would this even be recommended or is there some issue with this?


If your worried about swapping out to disk frequently (thrashing), creating a ram disk to take up more physical memory isn't going to help. The only solution to run more programs simultanously and not swapping pages between main memory and disk to invest in more memory. If your running a 64 bit OS, then buy all the RAM you think is sufficent (it's relativly cheap).

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I already know that I will be using a 256 Gb SSD, I won it in a programming competition. If I have a RAMDisk setup is it possible to mem-swap to both

Maybe, doesn't seem benifecal to swap memory pages in the same memory, plus it leads to a hierarchy of dirty pages starting from main memory -> ram disk -> disk. If you are using a SSD, you should'nt be concerning your self with ramdisk, you should invest in enough RAM to make the chances of swapping out to disk (or SSD) improbable.

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If so is it possible to do in such a way that more frequently accessed pages will be on the RAMDisk rather than the SSD

It would be a simple LRU replacement policy, which most systems (somewhat) implement already between main memory and disk; this, however is apart of the system's architecture. If swapping between ramdisk and SSD is possible, this is the replacement policy that would be implemented (or the simplified clock policy)

Quote

If I do have a setup where mem-swaping can be distributed and prioritized across a RAMDisk and a SSD what kind of performance hit would a single 64-bit application take by having to mem-swap to the RAMDisk sometimes?

Usually, adding a faster memory into the heiarchy (such as ram disk) won't hurt performance since the average memory access time will decrease, however it will leave you with more inconsistancies in your memory heirarchy. Depending on which type of write policy is implemented between ramdisk and ssd (write back, write through), there will be more sync writes to keep all memory consistant, that's the only performance hit I can think of.

This post has been edited by jjl: 24 June 2013 - 03:41 PM

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