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Why Use Virtual Memory In Modern Systems?
Posted by
timothy
on Thu Dec 04, 2008 06:00 PM
from the virtually-useless dept.
from the virtually-useless dept.
Cyberhwk writes "I have a system with Windows Vista Ultimate (64-bit) installed on it, and it has 4GB of RAM. However when I've been watching system performance, my system seems to divide the work between the physical RAM and the virtual memory, so I have 2GB of data in the virtual memory and another 2GB in the physical memory. Is there a reason why my system should even be using the virtual memory anymore? I would think the computer would run better if it based everything off of RAM instead of virtual memory. Any thoughts on this matter or could you explain why the system is acting this way?"
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You mean physical memory right :-) (Score:5, Informative)
You must be confused about virtual vs. physical memory. In modern processors, there is no penalty for using virtual memory, all translation from virtual to physical address space is done internal to the processor and you won't notice the difference.
So all the physical memory installed in your PC is used by the processor as one big pool of resources. Processes can think whatever they want and address huge memory spaces, that's all in virtual land. Virtual memory only starts impacting performance when pages are being swapped in and out, because all your processes need more resident memory than you actually have.
Swapping means accessing the disk and freezing the requesting process until its page of memory has arrived from the disk, which takes millions of processor cycles (a lifetime from the processor's point of view). It's not so bad if you swap once, as the processor can work on other processes while waiting for the data to arrive, but if all your programs keep pushing each other out of physical memory, you get thrashing and consider yourself happy if the mouse pointer is still responsive!
So you may want to change the title of your post to: "why use physical memory in modern systems?". I would point you to an article I wrote on that topic in 1990, but somehow I can't find a link to it on the web :-)
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Re:You mean physical memory right :-) (Score:5, Insightful)
You must be confused about virtual vs. physical memory.
Indeed. When I read this story my knee jerk reaction was "please be gentle." And thankfully the first +5 post on this story is informative and helpful and relatively kind.
... 2 ... 1 ...
I fear the "turn off your computer, put it in a box and mail it back to the manufacturer" hardcore hardware experts that are going to show up in 3
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Re:You mean physical memory right :-) (Score:5, Funny)
Gentle answers is what 6 years in customer support teaches you.
That, or hating everyone ;-)
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Re:You mean physical memory right :-) (Score:5, Funny)
Gentle answers is what 6 years in customer support teaches you.
That, or hating everyone ;-)
That kind of attitude really pisses me off! ;-)
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Re:You mean physical memory right :-) (Score:5, Funny)
So do I really only need 640k of physical memory if I have a modern system?
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Re:You mean physical memory right :-) (Score:5, Insightful)
Actually no the author was correct in Microsoft's Windows' terms. This is the exact text used in System Properties -> Advanced tab under Virtual memory:
"A paging file is an an area on the hard disk that Windows uses as if it were RAM."
You might think well they said paging file not virtual memory, well click on Change button and you'll see the dialog pop up named "Virtual Memory" of which you can specify multiple paging files on multiple drives if you wanted to. Defaulted to a single paging file on the C:\ or boot drive. So blame Microsoft for the confusing use of virtual memory and paging file back and forth. I guess they mean by virtual memory as in the collection usage of paging files after the fact (for those situations where there's more than one paging file used, just like on Linux you can have more than one swap file in use).
Anyway, I too have seen Windows 2000 and XP just love to make heavy use of the paging file even though there is clearly enough physical memory available. Some friends of mine have even disabled Windows from using a paging file completely, at first you will get a warning about it, but other than that they have reported better system performance and no draw backs noticed since then. This is on systems with at least 3GB of RAM.
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Re:You mean physical memory right :-) (Score:5, Informative)
Sorry, got to correct the path to where exactly I got that quote from:
System Properties -> Advanced -> Performance area, click Settings -> Advanced tab (on Windows XP, as for 2000 its the default tab).
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rephrasing his question charitably... (Score:5, Interesting)
I'd assume what he's asking is: in modern systems where the amount of physical RAM is considerably larger than what most people's programs in total use, why does the OS ever swap RAM out to disk?
The answer is basically to free up RAM for disk cache, based on a belief (sometimes backed up by benchmarks) that for typical use patterns, the performance hit of sometimes having to swap RAM back into physical memory is outweighed by the performance gain of a large disk cache.
Of course, OS designers are always revisiting these assumptions---it may be that for some kinds of use patterns using a smaller disk cache and swapping RAM out to disk less leads to better performance, or at least better responsiveness (if that's the goal).
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Re:rephrasing his question charitably... (Score:5, Informative)
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Re:rephrasing his question charitably... (Score:5, Interesting)
The question, though, is how is the reduction in disk cache size resulting from having no virtual memory to speak of affecting your runtime? Rather than seeing it all at once, like when you swap back in Firefox, are you taking longer to navigate directories because it has to read them in every single time? And when you're using firefox, does it take longer to check its disk cache? Are you saving 2 seconds when you switch applications by losing 60 seconds over the course of 10 minutes as you're actually using an individual application?
Saving the 60 seconds (perhaps at the expense of the 2 seconds) is exactly what the block cache is trying to do for you. Whether it's succeeding or not, or how well, is a different question. :-)
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Virtual Memory v Paging (Score:5, Informative)
thinks "Virtual Memory" is the same thing as paging...
Mac Classic (OS 8 for sure) used the term "Virtual Memory" the same way Windows today uses "Page File" or unix uses "swap", so you can at least understand why some people might be confused by this.
db
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Memory exists to be used (Score:5, Insightful)
Memory exists to be used. If memory is not in use, you are wasting it. The reality is that your system will operate with higher performance if unused data is paged out of RAM to disk and the newly freed memory is used for additional disk caching. Vista's memory manager is actually reasonably smart and will only page data out to disk when it really won't be used, or you experience an actual low-memory condition.
Re:Memory exists to be used (Score:5, Interesting)
I've known this argument for many years, I just don't think it applies anymore. The extra disk cache doesn't really help much, and what ends up happening is that I come in to work in the morning, unlock my work XP PC, and I sit there for 30 seconds while everything gets slowly pulled of the disk. XP thought it would be wise to page all that stuff out to disk, after all, I wasn't using it. But why would I care about the performance of the PC when I'm not actually using it?
At the very least, the amount of swap should be easily configurable like it is in Linux. I haven't actually used a swap partition in Linux for years, preferring instead to have 6 or 8gb of RAM, which is now cheap.
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Re:Memory exists to be used (Score:5, Informative)
At the very least, the amount of swap should be easily configurable like it is in Linux. I haven't actually used a swap partition in Linux for years, preferring instead to have 6 or 8gb of RAM, which is now cheap.
It is, (Right-click "My Computer")->Properties, "Advanced" tab, "Settings" under Performance, "Advanced" tab, "Change" under "Virtual memory". Almost as easy as "dd if=/dev/zero of=swapfile bs=1G count=1; swapon swapfile", spclly if u cant spel cuz u txt 2 much.
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Agreed (Score:5, Interesting)
Linux kernel maintainer Andrew Morton sets his swappiness [kerneltrap.org] to 100 (page as much physical memory as you can, the opposite of this Ask-Slashdot's desires), which he justified in an interview (see above link) by saying:
My point is that decreasing the tendency of the kernel to swap stuff out is wrong. You really don't want hundreds of megabytes of BloatyApp's untouched memory floating about in the machine. Get it out on the disk, use the memory for something useful.
Of course, there's another view, also presented at the above kerneltrap article: If you swap everything, you'll have a very long wait when returning to something you haven't touched in a while.
If you have limited resources, milk the resources you have plenty of; workstations should have high swappiness while laptops, who suffer in disk speed, disk capacity, and power, are probably better suited with lower swappiness. Don't go crazy, though ... swappiness = 0 is the same as running swapoff -a and will crash your programs when they need more memory than is available (as the kernel isn't written for a system without swap).
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The real answer is (Score:5, Funny)
[/tinfoil hat]
Would it help if (Score:5, Funny)
Then all your virtual memory is in RAM.
I'll leave it to someone else to explain why that isn't a good idea.
Re:Would it help if (Score:5, Interesting)
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Turn it off, then! (Score:5, Insightful)
We who know what we are doing are free to take the risk of running our computers without a swapfile.
Most people are not in a position where they can be sure that they will never run out of physical memory. Because of that, all operating systems for personal computers set up a swapfile by default: It's better for joe average computer owner to complain about a slow system than for him to lose his document when the system crashes because he filled up the physical memory (and there is no swap file to fall back on).
Why use a file system? (Score:5, Interesting)
The other extreme point of view is that modern systems should only have virtual memory and, instead of having an explicit file system, treat mass storage as a level-4 cache. In fact, systems that support mmap(2) do this partially.
The idea here is that modern memory management is actually pretty good, and that it's best to let the OS decide what to keep in RAM and what to swap out, so that issues like prefetching can be handled transparently.
Multics (Score:5, Insightful)
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I prefer none. (Score:5, Insightful)
This should generate some polarized discussion.
There are two camps of thought.
One will insist that, no matter how much memory is currently allocated, it makes more sense to swap out that which isn't needed in order to keep more free physical ram. They will argue until they are blue in the face that the benefits of doing so are good.
Essentially - your OS is clever and it tries pre-emptively swap things out so the memory will be available as needed.
The other camp - and the one I subscribe to - says that as long as you have enough physical ram to do whatever you need to do - any time spent swapping is wasted time.
I run most of my workstations (Windows) without virtual memory. Yes, on occasion, I do hit a "low on virtual memory error" - usually when something is leaky - but I prefer to get the error and have to re-start or kill something rather than have the system spend days getting progressively slower, slowly annoying me more and more, and then giving me the same error.
This is not to say that swap is bad, or that it shouldn't be used - but I prefer the simpler approach.
Swap is expected, so without it, you crash. (Score:5, Informative)
I recall back in 2002 or so, a friend of mine maxed out his Windows XP system with 2gb of memory. Windows absolutely refused to turn off paging (swap), forcing him to whatever the minimum size was. The solution? He created a RAMdisk and put the paging file there.
On Linux (and other modern systems, perhaps now including Windows), you can turn off swap. However, the Linux kernel's memory management isn't so great at the situation you hit when you need more memory than you have, but you can't swap. Usually, the memory hog crashes as a result (thankfully, Firefox now has session restore). I might be slightly out of date on this one.
A well-tweaked system still has swap (in nontrivial amounts), but rarely uses it. Trust me, you can afford losing the few gigabytes from your filesystem. Again in Linux, /proc/sys/vm/swappiness [kerneltrap.org] can be tweaked to a percentage reflecting how likely the system is to swap memory. Just lower it. (Though note the cons to this presented at the kerneltrap article above.) My workstation currently has an uptime of 14 days, a swappiness of 60, and 42/1427 megs of swap in use as opposed to the 1932/2026 megs of physical memory in use at the moment.
This is summarized for Windows and Linux on Paging [wikipedia.org] at Wikipedia.
Re:Vista reserves 1 GB (Score:5, Informative)
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Re:Vista reserves 1 GB (Score:5, Informative)
I think he is referring to the userspace/kernelspace split in Windows NT. On 32bit Windows XP, by default, the userspace (ring3) will have at most 2 GB of the physical RAM, and the kernel space would get the rest (some of it paged and some of it not). On systems with more than 3G of RAM (a lot by 2002 standards), it was kinda pointless to reserve that much for the kernel space, so they added a boot.ini flag that changed the split to _AT_MOST_ 3GBytes for the userspace and the rest for kernel space.
In Vista the split for 3G/1G of RAM is default. Actually on a system with 4G of RAM running in 32bit mode, you can't use all of them even if you try (in Windows XP), because right under the 4G limit you would have the PCI memory address mappings, that can be as large as 512M for a common video card with half a gig of RAM. Add to that the RAID controllers and the other hardware, and you have about 800megs of RAM unused because they can't be addressed, as their address-space is used by the installed devices.
I think that http://support.microsoft.com/kb/823440/ [microsoft.com] and http://support.microsoft.com/kb/171793/ [microsoft.com] should describe what I'm talking about pretty clearly.
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