Why Use Virtual Memory In Modern Systems?

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?"

Comments

Why use a file system?

[chrylis]

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.

File - Save

[Anonymous Coward]

For that matter, why do we even need to explicitely "save" anymore? Why does the fact that Notepad has 2KB of text to save prevent the shutdown of an entire computer? Just save the fecking thing anywhere and get on with it! Modern software is such a disorganized mess.

Re:Memory exists to be used

[etymxris]

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.

rephrasing his question charitably...

[Trepidity]

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).

Re:rephrasing his question charitably...

[martyros]

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. :-)

Re:rephrasing his question charitably...

[Solandri]

The problem I noticed with XP (dunno if Vista does the same) is that it doesn't seem to give running apps priority over disk cache. So if you have your browser in the background and hit a lot of files (e.g. a virus scan), the browser would get paged to disk and would take forever to bring back to the foreground.

What would be great is a setting like, "disk cache should never exceed 256 MB unless there is free RAM". In other words, if the total memory footprint of the OS and my running apps is less than my physical RAM minus 256 MB, they will never be swapped to disk. As I start approaching the limit, the first thing to be scaled back should be disk cache. Disk cache >256 MB will not be preserved by swapping my apps to disk.

As it is, I set XP's swapfile manually to 128 MB (any smaller and I would get frequent complaints about it being too small even though I have 3 GB of RAM). If it really needs more memory, it will override my setting and increase the swapfile size. But 99% of the time this limits the amount of apps XP can swap to disk to just 128 MB, which for me results in a much speedier system.

Re:rephrasing his question charitably...

[lgw]

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.

We're rapidly getting to the point where there's enough RAM for not only all the programs you're running, but all of the disk that those programs will access! Paging memory out to disk just doesn't make much sense anymore. I've run WIndows with no page file since Win2000 came out, and never had a problem with that.

My current (non-gaming) desktop box has 8GB of RAM, and cost me about $1000. I rarely use that much memory for the combined total of apps, OS footprint, and all non-streaming files (there's no point in caching streaming media files on a single-user system, beyond maybe the first block).

I expect my next $1000 system in a few years will have 64GB of RAM, at which point there really will be no point in using a page file for anything. And with a solid-state hard drive, I'm not sure there will be any point in read caching either (though write caching will still help I guess).

Re:rephrasing his question charitably...

[Mprx]

It might save 60 seconds, but it's saving the wrong 60 seconds. I'm not going to notice everything being very slightly faster, but I'll notice Firefox being swapped back from disc. I only care how long something takes if I have to wait for it.

Kernel developers seem to mostly care about benchmarks, and interactive latency is hard to benchmark. This leads to crazy things like Andrew Morton claiming to run swappiness 100 (swappiness 0 is the only acceptable value IMO if you need swap at all). I don't use swap, and with 4GB ram I never need it.

More than just memory management

[frog_strat]

Virtual memory is now used for little tricks, in addition to providing more memory than is physically available.

One example is ring transitions into kernel mode which start out as exceptions. (Everyone seems to have ignored call gate, the mechanism Intel offered for ring transitions).

Another is memory mapped pointers. It is cool to be able to increment a pointer to file backed ram and not have to care if it is in ram or not.

Maybe the OP is onto something. Imagine writing Windows drivers without having to worry about IRQL and paging.

Re:Would it help if

[Changa_MC]

I know it's not a good idea now, but this was seriously a great trick under win98. Win98 Recognized my full 1GB of RAM, but seemed to want to swap things to disk rather than use over 256MB of RAM. So I just created a RAM disk using the second 512MB of RAM, and voila! Everything ran much faster. When everything is broken, bad ideas become good again.

Agreed

[Khopesh]

Linux kernel maintainer Andrew Morton sets his swappiness 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).

Re:You mean physical memory right :-)

[Reziac]

I've been running without a pagefile, in all versions of Windows, for about 10 years now -- on any machine with more than 512mb.

The only drawback is that a few stupid Photoshop plugins whine and refuse to run, because if they don't see a pagefile, they believe there is "not enough memory" -- a holdover from the era when RAM was expensive and the pagefile was a busy place. Sometimes I think about making a very small pagefile just for them, but have never actually got around to doing it.

Can't hibernate

[anomaly]

Windows makes me CRAZY about this. the OS is internally configured to use an LRU algorithm to aggressively page.

("Technical bastards" who question my use of paging and swap interchangeably in this post can send their flames to /dev/null \Device\Null or NUL depending on OS)

What I found when disabling paging on an XP pro system with 2GB RAM is that the system performance is explosively faster without the disk IO.

Even an *idle* XP pro system swaps - explaining the time it takes for the system to be responsive to your request to maximize a window you have not used in a while.

I was thrilled to have a rocket-fast system again - until I tried to hibernate my laptop. Note that the hibernation file is unrelated to the swap/paging space.

The machine consistently would blue screen when trying to hibernate if swap/paging was disabled. Enabling swap enabled the hibernation function again. Since reboots take *FOREVER* to reload all the crap that XP needs on an enterprise-connected system - systems mangement, anti-virus agent, software distribution tool, and the required ram-defragger which allows XP to "stand by" when you've got more than 1GB of RAM, plus IM, etc

I reboot as infrequently as possible and consider "stand by" and "hibernate" required functions. As a result, I live with XP and paging enabled, and tolerate the blasted system "unpaging" apps that have been idle a short time.

Poo!

Re:I prefer none.

[Just Some Guy]

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.

Most of the people in this camp are coming from a Unix background where this is actually implemented effectively. For example, the FreeBSD machine next to my desk has 6GB of RAM, but even with about 3GB free, I'm currently about 1GB into my 16GB of swap. (Why 16? Because it's bigger than 6 but still a tiny part of my 750GB main drive.)

FreeBSD, and I assume most other modern Unixes, will copy idle stuff from RAM to swap when it's sufficiently bored. Note that it doesn't actually delete the pages in physical memory! Instead, it just marks them as copied. If those processes suddenly become active, they're already in RAM and go on about their business. If another process suddenly needs a huge allocation, like if my site's getting Slashdotted, then it can discard the pages in RAM since they've already been copied to disk.

That is why many Unix admins recommend swap. It helps the system effectively manage its resources without incurring a penalty, so why wouldn't you?

It's my understanding that Windows never managed to get this working right, so a lot of MS guys probably prefer to avoid it.