Memory Swap Issues

Swap space on a hard drive has been with Linux from almost the beginning. Ram was very expensive and swap let things like kernel building happen. However, the use of swap to disk during audio tasks is the death of the track being recorded, or a noticeable audio glitch in a live performance.

What can be done?

  • Memory Locking: UbuntuStudio already gives audio applications the right to use "memory locking". That is, they can tell the OS their data in RAM must not be swapped out. Good first step. However, I have found that one audio app that has no memlock will freeze Jackd while it cycles it's ram back in. Jackd is trying to output a bit of ram that is on disk just now... and all sound stops.

  • Swappiness: This is a variable the kernel uses to decide how full the memory has to be before it starts swapping out bits of memory that are not being used right now. It is based on what is left. The standard Ubuntu setting (an most others) is 60. That is when there is 60% ram left, the kernel starts swapping things out. This is a great setting for a server (the mysql people seem to disagree) but not for a desktop and really bad for audio work. The Linuxmusicians page recommends a setting of 10 and some people recommend a setting of 1 just for normal desktop work. This does seem to help.

  • zram: This is a utility that takes some of the ram space and makes a compressed swap space in ram. It can take a 2Gig ram and make it look like 3Gig or more. This compressed swap space is much faster than disk swap and makes the desktop more responsive even if an app has been moved to this ram swap. However, for audio even this much faster swap is too slow and there can be a discernible audio drop out if one of the audio samples happens to be swapped out. This could be a partial answer for things like qjackctl that can be swapped out but need to respond to user input quite quickly for things like making connection changes.

  • swapoff: How about just turning swap off? There are two reasons not to turn off swap. If Linux goes into hybernation it uses disk swap to hold it's state while the power is off, so no swap means no hybernation. Also, when ram is full, the kernel starts Killing apps or won't let you start them. My answer? Lets say I have 1Gig of Ram and 1Gig of swap (actually that is what I have). That is 2Gig, after that the kernel will shut things off anyway... but I will probably kill stuff before then because of swapping death. I could just go out and buy another Gig of Ram and have a responsive system till I run out at 2Gig. My thought is that it is better for the kernel not to let me start working than for swap to spoil the "perfect take".

  • Issues with more than one processor: when there are more than one processor in separate sockets with separate ram (NUMA), there are other issues. Linux tends to use up the ram on one processor and start swapping even though there is still free ram at another processor. This can make a low "swappiness" value look like it is not working.

  • More Ram: With Ram prices today, this should be a no brainer. Get more Ram! This should probably be the first tweak in the list.

  • memlock wrapper: A memlock wrapper to use on audio apps without their own memlock... Does such a thing exist?

My guess is that more Ram and swappiness set to 1 or even 0 would be the best start. Trying to use zram may be a help too if the user is careful to keep essential audio tasks from going in there. I don't know if this is possible. Turning swap off while recording might be something to try too. More testing needed.

Memory Leaks

While looking at swap another issue was found. Some programs have memory leaks. That is a program keeps asking for some memory, but never gives up memory it no longer needs. Swappiness can be set to zero, but if a program has a memory leak, at some point the Linux kernel will push even programs that are idle for only a short time into swap so it can grant memory to the leaking program.

  • Please report any such program to it's developer.
  • Use only the applications you need.
  • Choose screen blanking over screen savers.

UbuntuStudio/audio-settings/swap (last edited 2012-05-24 13:37:14 by len-ovenwerks)