Tuning Linux VM swapping

Lank writes "Kernel developers started discussing the pros and cons of swapping to disk on the Linux Kernel mailing list. KernelTrap has coverage of the story on their homepage. Andrew Morton comments, '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.' Personally, I just try to keep my memory usage below the physical memory in my machine, but I guess that's not always possible..."

Comments

It doesn't work that way

[Pharmboy]

Personally, I just try to keep my memory usage below the physical memory in my machine, but I guess that's not always possible..."

I keep my memory usage much below the total ram on the servers, but in real life, the machine still swaps. This is because even tho the machine NEVER needs more ram than is available at any given time, over a period of days, it will use more than the available ram. It caches out the old data that was used 12 hours ago.

Unless you reboot every day (as in a client machine) you will use swap on just about any machine. Using swap is not bad. Using swap for a currently running application is not so good. This isn't a bug, its a feature. Reading data from swap after it has been accessed is still faster than reading new data from the drives, especially if its a network drive.

What's wrong with many resident pages?

[YetAnotherName]

You really don't want hundreds of megabytes of BloatyApp's untouched memory floating about in the machine...

Why not? BloatyApp, if it's that bloaty is probably an object oriented program with template instantiation (or is by Micro$oft); these programs are notoriously huge, but also have notoriously poor locality of reference. The user will get better perceived response if you can keep more of BloatyApp resident.

If there's space in memory, I don't see the point of pre-emptively ejecting as many LRU pages of BloatyApp. (Of course, I haven't RTFA, but this is /. so you're not supposed to!)

Re:How fast is swapping really these days?

[Moderation abuser]

Well, disk access speed, say 5ms. RAM access speed 10ns so RAM is approx half a million times faster than disk.

Re:God no...

[petabyte]

Actually, you can change it on the fly with /proc/sys/vm/swappiness Increasing the number will increase the agressiveness of the swapout. Mr. Morton runs with his set at 100 (the max). 0, I believe would turn swap.

My kernel has autoswappiness enabled so it figures out the number on its own. I'm running at 64 ATM on a 256 Meg system (ram donations accepted) :).

My vote....

[tsmithnj]

is to do something like AIX does, where I can use "vmtune" to customize the percentages of memory I devote (hard or soft limit) to filesystem pages or computational pages. This way I can tune for my Bloatware, tune for file copying a la XP, or tune for my DBMS, whatever suits me.... The developers could take it one step further and provide a simple, understandable (as opposed to AIX's) interface for configuration......

Re:God no...

[The Spoonman]

Right-click My Computer -> Advanced -> Performance -> Advanced -> Memory Usage. Set to Programs. Now, click Change under Virtual Memory. Set your cache size small. For 1G of RAM, you prolly don't need a biggun. I usually set it to 100M for Inital and Max and then up it based on how often the machine swaps.

Then, make the following changes to the registry:

HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\ClearPageFileAtShutdown, set to 1. I don't shut my machine down very often, but occasionally XP will increase the size of the pagefile if it absolutely needs to depending on circumstances. This forces it back to the size you want it when you restart.

HKLM\System\CurrentControlSet\Control\FileSystem\N tfsDisable8dot3NameCreation, set to 1 ONLY IF YOU USE NO 16-BIT APPS ON YOUR MACHINE. Speeds up writes.

HKLM\System\CurrentControlSet\Control\FileSystem\N tfsDisableLastAccessUpdate, set to 1 if you don't care when files are accessed. This is rarely needed, and the setting speeds up writes.

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Contro l\Session Manager\Memory Management\IoPageLockLimit. Little more complex:

Set to 4096 if you've got more than 32M RAM

Set to 8192 if you've got more than 64M RAM

16384, 128M

32768, 160M

65536, 256M

131072, 512M

This changes the maximum number of bytes that can be locked for I/O operations. The default is 512Kb. While the above are the recommendations, I've found stepping down one level to provide the most performance for my needs, YMMV. (For example, I have 256M, but I set my IO limit to 32768.)

HKLM\System\CurrentControlSet\Control\Session Manager\Memory Management\DisablePagingExecutive, Set to 1 to disable paging of the kernel.

There, that wasn't so hard, was it? For those who want to flame that statement, keep in mind, that the information above is easier to find than some of the tuning suggestions I've heard for Linux. I've used Linux for 10 years, and only today heard about /proc/sys/vm/swappiness. Oh, and all of the above apply from at least NT4+.