6 ms·

This is with swap disabled.

by throwaway2048 7y ago

blattimwind 7y ago
Swap and paging are different concepts. The VM can page clean pages (mmap'd files, like most code your system is running) from disk in and out all day if it wants, in fact, when Linux is thrashing, that's exactly what it's doing.
AnssiH 7y ago
You can think of all your executables and shared libraries on disk as a kind of read-only swap. If the system is low on memory, a page of program code may be dropped from RAM and re-fetched from disk when it is needed again, i.e. when that section of the code is being executed again. (this effect is not limited to program code, though)
Iwan-Zotow 7y ago
Anything mmaped and read-only (SO included) could be and would be ditched
ploxiln 7y ago
It should have been clarified, because it is not obvious: The kernel can evict memory-mappings of executable files which are currently running. When they jump to a part of the executable that is no longer in memory, it can page that part back in from the executable file on disk. This is pretty cool. But when memory is very low, the kernel will evict practically all user-space executable mappings from memory, and will be reading back in and evicting back out executable file contents on practically every single context switch. It's just trying so hard to squeeze out some space to make its tasks fit in memory and complete successfully. I think this was the desired behavior of big-iron batch-processing in the 90s? Not sure why it has persisted so long. I'm a big fan of linux and this is my biggest pet-peeve.
- ajross 7y ago
  What's the other option, though? It gets a fault for a page somewhere, it needs memory, and it has none. You have to evict something. What's your choice if not swap or filesystem-backed pages? Systems have never operated well under true VM pressure. Not in the 90's, not now. When the working set goes beyond available memory performance falls off a cliff. And the report in the article doesn't seem to have a good comparison anyway. I mean, do we seriously believe that Windows or OS X can handle this condition (4G, no swap) better? I mean, swap is default for a reason.
  - throwaway2048 7y ago
    I'd much rather have OOMkiller trigger than have my system sit completely unresponsive for huge chunks of time, while I slowly struggle to kill something.
    - dahfizz 7y ago
      While this might add convenience when all you do is use a web browser, Linux is huge in the server space. Having the OS suddenly and somewhat unpredictability killing programs is absolutely not an option for most workloads.
      - quazeekotl 7y ago
        I don't think having a system sit completely unresponsive to anything for a huge amount of time is exactly optimal either.
      - _jal 7y ago
        Absolutely, which is why configurable knobs are things that exist. I already have to mess with the OOMkiller on my servers; I'm fine with fiddling with knobs on my desktop box too. This is Linux, after all. Wasn't one of the constant talking points in the war over the Init System That Shall Not Be Named that Linux needs to scale from embedded to big iron?
      - eikenberry 7y ago
        If you work in the cloud for any length of time you learn this is not true. Every system must be able to handle processes being killed unpredictably otherwise you will have problems. Better to deal with it and plan for it than expect a system to have 100% uptime. Because you never have 100% uptime.
      - TheDong 7y ago
        The oom killer already exists on servers and already can kill programs. If you want to turn off overcommit and have the system power off when it runs out of memory, the kernel allows that. Whatever knob they add will certainly be configurable, and ubuntu desktop can configure it one way while ubuntu server configures it the other, if it turns out people would prefer that. In practice, people running servers seem to want oom killers to kick in before the server barfs. One example of this is facebook's oomd [1]. I assure you, they're running that on their servers, not their web-browser-machines. [1]: https://github.com/facebookincubator/oomd https://github.com/facebookincubator/oomd
- rjmunro 7y ago
  > ... on practically every single context switch ... How frequently do context switches happen? Can this be reduced, so that there is less disk thrashing? I.e. whatever process needs lots of memory swapped in, just don't switch to it for up to maybe a few seconds, rather than milliseconds or whatever? This seems less bad than having the OOM killer just kill the thing.

This is with swap disabled.

Swap and paging are different concepts. The VM can page clean pages (mmap'd files, like most code your system is running) from disk in and out all day if it wants, in fact, when Linux is thrashing, that's exactly what it's doing.

AnssiH 7y ago

You can think of all your executables and shared libraries on disk as a kind of read-only swap. If the system is low on memory, a page of program code may be dropped from RAM and re-fetched from disk when it is needed again, i.e. when that section of the code is being executed again. (this effect is not limited to program code, though)

Iwan-Zotow 7y ago

Anything mmaped and read-only (SO included) could be and would be ditched

ploxiln 7y ago

It should have been clarified, because it is not obvious: The kernel can evict memory-mappings of executable files which are currently running. When they jump to a part of the executable that is no longer in memory, it can page that part back in from the executable file on disk. This is pretty cool. But when memory is very low, the kernel will evict practically all user-space executable mappings from memory, and will be reading back in and evicting back out executable file contents on practically every single context switch. It's just trying so hard to squeeze out some space to make its tasks fit in memory and complete successfully. I think this was the desired behavior of big-iron batch-processing in the 90s? Not sure why it has persisted so long. I'm a big fan of linux and this is my biggest pet-peeve.

ajross 7y ago
What's the other option, though? It gets a fault for a page somewhere, it needs memory, and it has none. You have to evict something. What's your choice if not swap or filesystem-backed pages? Systems have never operated well under true VM pressure. Not in the 90's, not now. When the working set goes beyond available memory performance falls off a cliff. And the report in the article doesn't seem to have a good comparison anyway. I mean, do we seriously believe that Windows or OS X can handle this condition (4G, no swap) better? I mean, swap is default for a reason.
- throwaway2048 7y ago
  I'd much rather have OOMkiller trigger than have my system sit completely unresponsive for huge chunks of time, while I slowly struggle to kill something.
  - dahfizz 7y ago
    While this might add convenience when all you do is use a web browser, Linux is huge in the server space. Having the OS suddenly and somewhat unpredictability killing programs is absolutely not an option for most workloads.
    - quazeekotl 7y ago
      I don't think having a system sit completely unresponsive to anything for a huge amount of time is exactly optimal either.
    - _jal 7y ago
      Absolutely, which is why configurable knobs are things that exist. I already have to mess with the OOMkiller on my servers; I'm fine with fiddling with knobs on my desktop box too. This is Linux, after all. Wasn't one of the constant talking points in the war over the Init System That Shall Not Be Named that Linux needs to scale from embedded to big iron?
    - eikenberry 7y ago
      If you work in the cloud for any length of time you learn this is not true. Every system must be able to handle processes being killed unpredictably otherwise you will have problems. Better to deal with it and plan for it than expect a system to have 100% uptime. Because you never have 100% uptime.
    - TheDong 7y ago
      The oom killer already exists on servers and already can kill programs. If you want to turn off overcommit and have the system power off when it runs out of memory, the kernel allows that. Whatever knob they add will certainly be configurable, and ubuntu desktop can configure it one way while ubuntu server configures it the other, if it turns out people would prefer that. In practice, people running servers seem to want oom killers to kick in before the server barfs. One example of this is facebook's oomd [1]. I assure you, they're running that on their servers, not their web-browser-machines. [1]: https://github.com/facebookincubator/oomd https://github.com/facebookincubator/oomd
rjmunro 7y ago
> ... on practically every single context switch ... How frequently do context switches happen? Can this be reduced, so that there is less disk thrashing? I.e. whatever process needs lots of memory swapped in, just don't switch to it for up to maybe a few seconds, rather than milliseconds or whatever? This seems less bad than having the OOM killer just kill the thing.