4 ms·

Swap actually increases system performance for many common workloads: https://chrisdown.name/2018/01/02/in-defence-of-swap.html https://chrisdown.name/2018/01/0

Swap actually increases system performance for many common workloads: https://chrisdown.name/2018/01/02/in-defence-of-swap.html https://chrisdown.name/2018/01/02/in-defence-of-swap.html

morelisp 4y ago
The basic argument is that swap can be preferrable to hard eviction for managing disk cache, which can be true, but disk-heavy workloads are less common than ever especially in general-use clusters. (The other argument is the usual mix of "but it can swap out unused areas" in which case you should fix the program to not allocate a ton of memory it doesn't need; and "but the OOM killer will come" and yes that's the entire point please move my process elsewhere).
- shawnz 4y ago
  > "but it can swap out unused areas" in which case you should fix the program to not allocate a ton of memory it doesn't need Isn't this basically asking every application to independently reimplement their own swap-style functionality? Surely it makes more sense to have swap as a system level feature that every application can take advantage of? That way the system has more information available to make swapping decisions since it can take into account the memory usage of all applications and drivers/etc together, and applications can take advantage of the most advanced swapping algorithms in the latest versions of the system without having to be individually updated.
  - morelisp 4y ago
    > Isn't this basically asking every application to independently reimplement their own swap-style functionality? No, it's asking for applications not to allocate a bunch of space they don't need in the first place.
    - shawnz 4y ago
      If you are suggesting that your applications are allocating a bunch of memory which they literally never use in any way, then I don't think that is really a common issue in practice. In practice I expect the typical scenario you're thinking of is applications are allocating a bunch of memory which they use only briefly or rarely and could easily fetch or recompute that data again later if needed. That's exactly the scenario which swap optimizes for. So why should every application individually implement logic to optimize for it? Fixing these kind of issues is not as simple as "just don't allocate the memory".
      - morelisp 4y ago
        > If you are suggesting that your applications are allocating a bunch of memory which they literally never use in any way, then I don't think that is really a common issue in practice. That is what I'm suggesting, and I suggest you look at how much space is wasted by startup-initialized data in libraries for features you'll never use, JIT representations that never get compiled for more than one shape, class metadata that never gets touched and vtables which don't get used, how easy it is in any GC language to accidentally keep a large buffer alive until the end of a lexical scope rather than its last real use, etc. etc. Today's programs are bloated as hell dude.
        shawnz 4y ago
        All these problems you're describing are way more complicated than "just don't allocate the memory" and would involve sophisticated logic to optimize (like predicting exactly what features are or are not going to be used by an application at start time without significantly increasing the startup latency, etc). Why not just let swap be that optimization?
        morelisp 4y ago
        Mediocrity uber alles forever!
        shawnz 4y ago
hedora 4y ago
Yes, but it helps for essentially zero large server workloads. (Which is why most big server deployments disable it.)
nisa 4y ago
This article is always thrown around if the topic comes up, at least in my experience it's wrong or at least not useful for my usecases (general purpose web hosting stuff in all kinds of flavours) - swap on spinning rust outright kills the system if memory is low. it's a full blown crash. - swap on ssd act's much the same - zram swap stalls and slows down the system for several minutes I just raise vm.min_free_kbytes to make the oom killer faster and disable swap. At least the machines don't crash hard this way.
- AshamedCaptain 4y ago
  You are either abusing the word "crash" or otherwise make a very dubious assertion here.
  - nisa 4y ago
    crash as in it's not recoverable using ssh - machine only responds to ping and needs a hard reset.
    - andrewjf 4y ago
      Which is exactly the situation you never, ever want a server in. So it’s actually worse than a “crash” Me and my team removed swap on tens of thousands of machines cause we were sick of dealing with this. We wanted the machines to fail hard and fast and not go into a state where it’s doing only a minuscule amount of actual work while trying to recover.
      - cosarara 4y ago
        And I think that, sadly, it's not even a full solution, because linux can manage to get thrashing even without swap. It pages in and out things like memory mapped files or the content of executables of stopped processes. See for instance: https://serverfault.com/questions/898388/how-to-prevent-kernel-i-o-thrashing-in-absence-of-swap-memory-on-linux https://serverfault.com/questions/898388/how-to-prevent-kern...
- 4y ago
throwaway09223 4y ago
It really doesn't. When the author article writes about swap helping "Under moderate/high memory contention" -- most people have already powercycled their device at this point (or monitoring has pulled the struggling node from production). It's utterly useless, due to the huge difference in latency between RAM and storage. On most production systems, memory allocation is roughly 1% system and 99% application (not counting temporary, evictable allocations like disk cache). Modern applications are not designed to have their pages swapped out to disk and it is not particularly helpful to swap the teeny tiny bits of OS. There are parts of this article which are true, but the overall picture it paints is not accurate and the conclusion that swap is helpful is not correct for high performance systems - both workstations and servers.

Swap actually increases system performance for many common workloads: https://chrisdown.name/2018/01/02/in-defence-of-swap.html https://chrisdown.name/2018/01/02/in-defence-of-swap.html

morelisp 4y ago

The basic argument is that swap can be preferrable to hard eviction for managing disk cache, which can be true, but disk-heavy workloads are less common than ever especially in general-use clusters. (The other argument is the usual mix of "but it can swap out unused areas" in which case you should fix the program to not allocate a ton of memory it doesn't need; and "but the OOM killer will come" and yes that's the entire point please move my process elsewhere).

shawnz 4y ago
> "but it can swap out unused areas" in which case you should fix the program to not allocate a ton of memory it doesn't need Isn't this basically asking every application to independently reimplement their own swap-style functionality? Surely it makes more sense to have swap as a system level feature that every application can take advantage of? That way the system has more information available to make swapping decisions since it can take into account the memory usage of all applications and drivers/etc together, and applications can take advantage of the most advanced swapping algorithms in the latest versions of the system without having to be individually updated.
- morelisp 4y ago
  > Isn't this basically asking every application to independently reimplement their own swap-style functionality? No, it's asking for applications not to allocate a bunch of space they don't need in the first place.
  - shawnz 4y ago
    If you are suggesting that your applications are allocating a bunch of memory which they literally never use in any way, then I don't think that is really a common issue in practice. In practice I expect the typical scenario you're thinking of is applications are allocating a bunch of memory which they use only briefly or rarely and could easily fetch or recompute that data again later if needed. That's exactly the scenario which swap optimizes for. So why should every application individually implement logic to optimize for it? Fixing these kind of issues is not as simple as "just don't allocate the memory".
    - morelisp 4y ago
      > If you are suggesting that your applications are allocating a bunch of memory which they literally never use in any way, then I don't think that is really a common issue in practice. That is what I'm suggesting, and I suggest you look at how much space is wasted by startup-initialized data in libraries for features you'll never use, JIT representations that never get compiled for more than one shape, class metadata that never gets touched and vtables which don't get used, how easy it is in any GC language to accidentally keep a large buffer alive until the end of a lexical scope rather than its last real use, etc. etc. Today's programs are bloated as hell dude.
      - shawnz 4y ago
        All these problems you're describing are way more complicated than "just don't allocate the memory" and would involve sophisticated logic to optimize (like predicting exactly what features are or are not going to be used by an application at start time without significantly increasing the startup latency, etc). Why not just let swap be that optimization?
        morelisp 4y ago
        Mediocrity uber alles forever!
        shawnz 4y ago

hedora 4y ago

Yes, but it helps for essentially zero large server workloads. (Which is why most big server deployments disable it.)

nisa 4y ago

This article is always thrown around if the topic comes up, at least in my experience it's wrong or at least not useful for my usecases (general purpose web hosting stuff in all kinds of flavours) - swap on spinning rust outright kills the system if memory is low. it's a full blown crash. - swap on ssd act's much the same - zram swap stalls and slows down the system for several minutes I just raise vm.min_free_kbytes to make the oom killer faster and disable swap. At least the machines don't crash hard this way.

AshamedCaptain 4y ago
You are either abusing the word "crash" or otherwise make a very dubious assertion here.
- nisa 4y ago
  crash as in it's not recoverable using ssh - machine only responds to ping and needs a hard reset.
  - andrewjf 4y ago
    Which is exactly the situation you never, ever want a server in. So it’s actually worse than a “crash” Me and my team removed swap on tens of thousands of machines cause we were sick of dealing with this. We wanted the machines to fail hard and fast and not go into a state where it’s doing only a minuscule amount of actual work while trying to recover.
    - cosarara 4y ago
      And I think that, sadly, it's not even a full solution, because linux can manage to get thrashing even without swap. It pages in and out things like memory mapped files or the content of executables of stopped processes. See for instance: https://serverfault.com/questions/898388/how-to-prevent-kernel-i-o-thrashing-in-absence-of-swap-memory-on-linux https://serverfault.com/questions/898388/how-to-prevent-kern...
4y ago

throwaway09223 4y ago

It really doesn't. When the author article writes about swap helping "Under moderate/high memory contention" -- most people have already powercycled their device at this point (or monitoring has pulled the struggling node from production). It's utterly useless, due to the huge difference in latency between RAM and storage. On most production systems, memory allocation is roughly 1% system and 99% application (not counting temporary, evictable allocations like disk cache). Modern applications are not designed to have their pages swapped out to disk and it is not particularly helpful to swap the teeny tiny bits of OS. There are parts of this article which are true, but the overall picture it paints is not accurate and the conclusion that swap is helpful is not correct for high performance systems - both workstations and servers.