5 ms·

i routinely see these days systems without or with very low swap. It is like swap has become faux pas. That is especially strange giving the SSD drives availab

by trhway 7y ago

i routinely see these days systems without or with very low swap. It is like swap has become faux pas. That is especially strange giving the SSD drives available on the machines. Gradual degradation of service vs. the service sudden disappearance and/or stall or heavy overprovisioning and still ... Also comes to mind - while not generic swap - kind of edge case, a modern version of swap, ie. extending virtual memory space onto flash storage - Facebook replacement of some RAM with NVM https://research.fb.com/wp-content/uploads/2018/03/reducing-dram-footprint-with-nvm-in-facebook.pdf https://research.fb.com/wp-content/uploads/2018/03/reducing-...

IshKebab 7y ago
Yeah because swap on Linux doesn't really work. When it is needed your system grinds to a halt.
- Dylan16807 7y ago
  I strongly disagree with that characterization. At some amount of active memory use Linux will grind to a halt. This happens with or without swap. It's a problem of how it handles low memory situations. But if you have swap, this point comes much later. Often many gigabytes of unimportant data can be moved out of memory. If you can swap out 6 gigabytes without causing problems, and you only needed an extra 4 gigabytes of ram, then swap saves your day with almost no downside.
- trhway 7y ago
  i would venture a guess that probably you saw something like for example large JVM being partially swapped on HDD - in a situation like that it isn't swap per.se. which is at fault, it is the fact that JVM memory access pattern - randomly jumping all over - is extremely unsuitable for HDD based swap.
  - neel_k 7y ago
    Actually, there is a critically bad interaction between the way that garbage collection algorithms work and the LRU heuristic for swap. Tracing garbage collection algorithms work by doing a graph traversal that traces all the live pointers starting from the "root set" (ie, pointers in registers and on the stack) to find all the live objects. Now, note that (a) the root set are the hottest objects in memory, and (b) they are the ones that gc algorithms touch first. As a result, when you are doing a gc, the LRU (least-recently-used) heuristic for managing swap ends up putting all the hottest data in the program onto disk. This is pretty much the pessimal thing for the kernel to do, and can tank performance by triple-digit factors. However, this is entirely fixable! The gc and the kernel need to cooperate to figure out what is okay to push to disk or not. Matthew Hertz, Yi Feng and Emery Berger have a really nice PLDI 2005 paper, Garbage Collection without Paging, which shows how you can make gc performance hundreds of times better with just a little kernel assistance. However, because kernel developers do not program in garbage collected languages (they program in C, because that's what the kernel is written in), they don't understand garbage collection and are hostile to kernel changes to support it. Emery Berger told me they tried and failed to get the Linux kernel to support their changes, and people who worked on .NET told me they had similar experiences with the Windows kernel team.
- lclarkmichalek 7y ago
  Your system would grind to a halt anyway; swap is simply giving the kernel more options to deal with the issue. Without swap the kernel has no options when it cmes to unreclaimable pages. Obligatory link: https://chrisdown.name/2018/01/02/in-defence-of-swap.html https://chrisdown.name/2018/01/02/in-defence-of-swap.html
- imtringued 7y ago
  I didn't enable swap for the first 6 months, every time I ran out of memory the system froze. Now I use swap and only the offending applications do actually freeze. There is a small problem though, one application in particular permanently captures the damn mouse in fullscreen mode so if it freezes I can't interact with the rest of the system with the mouse but this is purely a bug in the application, not a problem with the operating system.
  - majewsky 7y ago
    Well, it's more of a design problem of X11. Under Wayland, the compositor could offer a Secure Attention Key like Ctrl-Alt-Del that reclaims input focus from a fullscreen application. As far as I'm aware, such facilities do not exist in X11. When an application grabs the keyboard, it's grabbing the entire keyboard.
    - deleted 7y ago
      [deleted]
    - ratmice 7y ago
      Yes, the problem is that X11's login/lock screen is secured by grabbing the keyboard and mouse, taking away input focus from the login screen in many cases will allow you to bypass it entirely.
    - Crinus 7y ago
      Such facilities actually do exist in Xorg, they are just disabled by default. But you can configure the server with a shortcut key to release input grab.
nguoi 7y ago
When installing a new OS recently, I struggled to find a definitive view on what to do with swap. There's no data about it, just superstition, and the thread about this yesterday was full of people saying nobody uses it. Maybe I can invoke Cunningham's law? I use 16GB on an NVMe SSD, which is equal to the size of my RAM.
- majewsky 7y ago
  I've been told that 90s-era manuals for SAP systems recommended swap size = 2x RAM size. There's a story circulating in the office that someone remembered this rule in 2012-ish when setting up a bunch of fresh systems with 2 TB RAM each, causing the storage support team to wonder why the fresh filer was already full after just a few days.
  - kofejnik 7y ago
    https://utcc.utoronto.ca/~cks/space/blog/unix/TheLegendOfSwapSizing https://utcc.utoronto.ca/~cks/space/blog/unix/TheLegendOfSwa... tl;dr - 2x RAM was true in the early 90s
- mediaserf 7y ago
  Here is a really good article breaking down what happens in linux with regard to swap, albeit a bit dated (2011) https://www.linuxjournal.com/article/10678 https://www.linuxjournal.com/article/10678
thaeli 7y ago
There's common advice out there that putting swap on SSD is bad, because it wears out the disks faster.
reirob 7y ago
And there is Cassandra DB, that advices to deactivate swap. I am wondering what happens on servers, running several Java Services with Cassandra and without swap, once memory pressure gets high, even due to system caches, will it stall as well for an eternity?

by trhway 7y ago

IshKebab 7y ago

Yeah because swap on Linux doesn't really work. When it is needed your system grinds to a halt.

Dylan16807 7y ago
I strongly disagree with that characterization. At some amount of active memory use Linux will grind to a halt. This happens with or without swap. It's a problem of how it handles low memory situations. But if you have swap, this point comes much later. Often many gigabytes of unimportant data can be moved out of memory. If you can swap out 6 gigabytes without causing problems, and you only needed an extra 4 gigabytes of ram, then swap saves your day with almost no downside.
trhway 7y ago
i would venture a guess that probably you saw something like for example large JVM being partially swapped on HDD - in a situation like that it isn't swap per.se. which is at fault, it is the fact that JVM memory access pattern - randomly jumping all over - is extremely unsuitable for HDD based swap.
- neel_k 7y ago
  Actually, there is a critically bad interaction between the way that garbage collection algorithms work and the LRU heuristic for swap. Tracing garbage collection algorithms work by doing a graph traversal that traces all the live pointers starting from the "root set" (ie, pointers in registers and on the stack) to find all the live objects. Now, note that (a) the root set are the hottest objects in memory, and (b) they are the ones that gc algorithms touch first. As a result, when you are doing a gc, the LRU (least-recently-used) heuristic for managing swap ends up putting all the hottest data in the program onto disk. This is pretty much the pessimal thing for the kernel to do, and can tank performance by triple-digit factors. However, this is entirely fixable! The gc and the kernel need to cooperate to figure out what is okay to push to disk or not. Matthew Hertz, Yi Feng and Emery Berger have a really nice PLDI 2005 paper, Garbage Collection without Paging, which shows how you can make gc performance hundreds of times better with just a little kernel assistance. However, because kernel developers do not program in garbage collected languages (they program in C, because that's what the kernel is written in), they don't understand garbage collection and are hostile to kernel changes to support it. Emery Berger told me they tried and failed to get the Linux kernel to support their changes, and people who worked on .NET told me they had similar experiences with the Windows kernel team.
lclarkmichalek 7y ago
Your system would grind to a halt anyway; swap is simply giving the kernel more options to deal with the issue. Without swap the kernel has no options when it cmes to unreclaimable pages. Obligatory link: https://chrisdown.name/2018/01/02/in-defence-of-swap.html https://chrisdown.name/2018/01/02/in-defence-of-swap.html
imtringued 7y ago
I didn't enable swap for the first 6 months, every time I ran out of memory the system froze. Now I use swap and only the offending applications do actually freeze. There is a small problem though, one application in particular permanently captures the damn mouse in fullscreen mode so if it freezes I can't interact with the rest of the system with the mouse but this is purely a bug in the application, not a problem with the operating system.
- majewsky 7y ago
  Well, it's more of a design problem of X11. Under Wayland, the compositor could offer a Secure Attention Key like Ctrl-Alt-Del that reclaims input focus from a fullscreen application. As far as I'm aware, such facilities do not exist in X11. When an application grabs the keyboard, it's grabbing the entire keyboard.
  - deleted 7y ago
    [deleted]
  - ratmice 7y ago
    Yes, the problem is that X11's login/lock screen is secured by grabbing the keyboard and mouse, taking away input focus from the login screen in many cases will allow you to bypass it entirely.
  - Crinus 7y ago
    Such facilities actually do exist in Xorg, they are just disabled by default. But you can configure the server with a shortcut key to release input grab.

nguoi 7y ago

When installing a new OS recently, I struggled to find a definitive view on what to do with swap. There's no data about it, just superstition, and the thread about this yesterday was full of people saying nobody uses it. Maybe I can invoke Cunningham's law? I use 16GB on an NVMe SSD, which is equal to the size of my RAM.

majewsky 7y ago
I've been told that 90s-era manuals for SAP systems recommended swap size = 2x RAM size. There's a story circulating in the office that someone remembered this rule in 2012-ish when setting up a bunch of fresh systems with 2 TB RAM each, causing the storage support team to wonder why the fresh filer was already full after just a few days.
- kofejnik 7y ago
  https://utcc.utoronto.ca/~cks/space/blog/unix/TheLegendOfSwapSizing https://utcc.utoronto.ca/~cks/space/blog/unix/TheLegendOfSwa... tl;dr - 2x RAM was true in the early 90s
mediaserf 7y ago
Here is a really good article breaking down what happens in linux with regard to swap, albeit a bit dated (2011) https://www.linuxjournal.com/article/10678 https://www.linuxjournal.com/article/10678

thaeli 7y ago

There's common advice out there that putting swap on SSD is bad, because it wears out the disks faster.

reirob 7y ago

And there is Cassandra DB, that advices to deactivate swap. I am wondering what happens on servers, running several Java Services with Cassandra and without swap, once memory pressure gets high, even due to system caches, will it stall as well for an eternity?