6 ms·

How commonly do people need to use swap in the first place? On my current computer, I don't even have a swap partition because I've never gotten close to fillin

by snazz 7y ago

How commonly do people need to use swap in the first place? On my current computer, I don't even have a swap partition because I've never gotten close to filling my 16GB, even with a big virtual machine and hundreds of browser tabs. In my experience on desktop systems, it just makes your computer painfully slow when you write a program that allocates far too much memory. Without swap, the OOM killer gets involved much earlier and you don't have to do a hard power cut to stop the disk from thrashing.

cesarb 7y ago
I just opened the site of the biggest retailer in my country, and looked at one entry-level laptop: it has 2GB of RAM. Not everybody has a high-end computer.
- sjwright 7y ago
  A Chromebook?
  - saagarjha 7y ago
    My $150 Chromebook has twice that; most computers these days tend to ship with more than than I think.
  - cesarb 7y ago
    No, a normal laptop from a local brand.
haneefmubarak 7y ago
When done well, swap can be pretty useful. I have a Fall 2019 MacBook Pro maxed out on CPU (8 cores) and RAM (64 GB) - MacOS still regularly compresses (highest I've seen so far: 12 GB) and/or swaps (highest I've seen so far: 16 GB) relatively unused memory to make more space for other applications and hot caches (which are still useful even with an PCIe SSD). Keep in mind that everything feels fully snappy and responsive while this is all happening.
barrkel 7y ago
If you don't have enough memory for a single app, then your life is completely miserable - in that case, swapping is almost indistinguishable from crashed. But swap can be the difference between being able to run one app at a time, vs run three or four, as long as you only have one in the foreground at a time. Swap doesn't play well with GC, or pointerful programming languages generally. Swap is a latency hit every time you touch an evicted page, and the more pointers your structures have, the more frequently that occurs. In older times, when memory needed to be conserved, GC was less frequently used, and as a consequence of the accounting overheads of manual memory allocation, pointerful data structures weren't quite as common. Instead, you'd have pre-allocated arrays which supported up to N items of a particular type (and tough luck if you wanted more), or in more sophisticated applications, arena allocators. In such systems, the working set of memory is more likely to be contiguous; paging works better because every page load is more likely to bring in relevant data. Switching between two open applications was the moment where you'd typically hit swap; that, and every now and then when you touched an area of the app you hadn't in a while, or jumped to a distant area of the document you were working on. The HDD light would come on, and you'd have to wait a few seconds while the computer ground through (HDDs being audible, some much louder than others) before you'd switch. At some point in the mid 2000s, GC languages tipped the balance, especially JS. The browser became the main application, and wouldn't tolerate swap well. It became a mini OS unto itself, and had to balance the competing needs of different tabs. Then mobile exploded, and didn't use swap at all in order to compete with the iPhone, remarkable for its smooth animations. If there was to be latency, the application had to hide it; OS-level introduced latency on a page fault wasn't good enough, because if it happened on the UI thread, you dropped frames. And of course swap never worked well on servers with frequent requests. Swapping on a server is a great way to back up all your queues.
saalweachter 7y ago
What's hilarious to me, growing up in the era of swap, is that now you can have so much RAM that you can start caching your disk to RAM for a little extra performance (well, we've kind of moved past that, too, since SSDs are so much faster than spinny disks).
- rcxdude 7y ago
  In fact, it's often worth moving some stuff in RAM to swap in order to have more space for disk cache (if you ever max out the RAM on a linux machine without swap you'll find it's a miserable experience because of the lack of cache and the fact that code from executables just gets memory mapped into the process address space).
shawnz 7y ago
By disabling swap, you are taking away your operating system's ability to trade less-frequently-used parts of memory for extra disk cache, which could hinder overall performance even if you have an excess of physical memory for your use case.
- colejohnson66 7y ago
  Honest question: what’s the difference (in terms of apps being killed) between 16 GB of RAM and 8 GB with an 8 GB swap?
  - shawnz 7y ago
    There is no difference in terms of how much memory apps will be able to allocate before getting killed. But there will be a significant performance difference once apps start using more memory than the 8GB that is physically available (of course).
- garaetjjte 7y ago
  The problem is, system cannot know when you're going to use it. If application is swapped out and you use it after several hours, there would be excruciatingly long delay when it restores from spinning HDD disk. If I have reasonable amount of RAM I definitely prefer not to have swap.
  - shawnz 7y ago
    Even if that means lower throughput from all your other apps over that several hour period? That is worth not having to wait a few extra seconds to go back to the less-used app?
    - garaetjjte 7y ago
      Depends, but usually not. Harder to notice minimally better performance than to be annoyed by 2 second lag on application switch. ("this computer have 32GB RAM and it's trashing disk! grr")
garaetjjte 7y ago
>you don't have to do a hard power cut to stop the disk from thrashing You can rescue from that situation by invoking OOM killer from SysRq magic key.

by snazz 7y ago

cesarb 7y ago

I just opened the site of the biggest retailer in my country, and looked at one entry-level laptop: it has 2GB of RAM. Not everybody has a high-end computer.

sjwright 7y ago
A Chromebook?
- saagarjha 7y ago
  My $150 Chromebook has twice that; most computers these days tend to ship with more than than I think.
- cesarb 7y ago
  No, a normal laptop from a local brand.

haneefmubarak 7y ago

When done well, swap can be pretty useful. I have a Fall 2019 MacBook Pro maxed out on CPU (8 cores) and RAM (64 GB) - MacOS still regularly compresses (highest I've seen so far: 12 GB) and/or swaps (highest I've seen so far: 16 GB) relatively unused memory to make more space for other applications and hot caches (which are still useful even with an PCIe SSD). Keep in mind that everything feels fully snappy and responsive while this is all happening.

barrkel 7y ago

If you don't have enough memory for a single app, then your life is completely miserable - in that case, swapping is almost indistinguishable from crashed. But swap can be the difference between being able to run one app at a time, vs run three or four, as long as you only have one in the foreground at a time. Swap doesn't play well with GC, or pointerful programming languages generally. Swap is a latency hit every time you touch an evicted page, and the more pointers your structures have, the more frequently that occurs. In older times, when memory needed to be conserved, GC was less frequently used, and as a consequence of the accounting overheads of manual memory allocation, pointerful data structures weren't quite as common. Instead, you'd have pre-allocated arrays which supported up to N items of a particular type (and tough luck if you wanted more), or in more sophisticated applications, arena allocators. In such systems, the working set of memory is more likely to be contiguous; paging works better because every page load is more likely to bring in relevant data. Switching between two open applications was the moment where you'd typically hit swap; that, and every now and then when you touched an area of the app you hadn't in a while, or jumped to a distant area of the document you were working on. The HDD light would come on, and you'd have to wait a few seconds while the computer ground through (HDDs being audible, some much louder than others) before you'd switch. At some point in the mid 2000s, GC languages tipped the balance, especially JS. The browser became the main application, and wouldn't tolerate swap well. It became a mini OS unto itself, and had to balance the competing needs of different tabs. Then mobile exploded, and didn't use swap at all in order to compete with the iPhone, remarkable for its smooth animations. If there was to be latency, the application had to hide it; OS-level introduced latency on a page fault wasn't good enough, because if it happened on the UI thread, you dropped frames. And of course swap never worked well on servers with frequent requests. Swapping on a server is a great way to back up all your queues.

saalweachter 7y ago

What's hilarious to me, growing up in the era of swap, is that now you can have so much RAM that you can start caching your disk to RAM for a little extra performance (well, we've kind of moved past that, too, since SSDs are so much faster than spinny disks).

rcxdude 7y ago
In fact, it's often worth moving some stuff in RAM to swap in order to have more space for disk cache (if you ever max out the RAM on a linux machine without swap you'll find it's a miserable experience because of the lack of cache and the fact that code from executables just gets memory mapped into the process address space).

shawnz 7y ago

By disabling swap, you are taking away your operating system's ability to trade less-frequently-used parts of memory for extra disk cache, which could hinder overall performance even if you have an excess of physical memory for your use case.

colejohnson66 7y ago
Honest question: what’s the difference (in terms of apps being killed) between 16 GB of RAM and 8 GB with an 8 GB swap?
- shawnz 7y ago
  There is no difference in terms of how much memory apps will be able to allocate before getting killed. But there will be a significant performance difference once apps start using more memory than the 8GB that is physically available (of course).
garaetjjte 7y ago
The problem is, system cannot know when you're going to use it. If application is swapped out and you use it after several hours, there would be excruciatingly long delay when it restores from spinning HDD disk. If I have reasonable amount of RAM I definitely prefer not to have swap.
- shawnz 7y ago
  Even if that means lower throughput from all your other apps over that several hour period? That is worth not having to wait a few extra seconds to go back to the less-used app?
  - garaetjjte 7y ago
    Depends, but usually not. Harder to notice minimally better performance than to be annoyed by 2 second lag on application switch. ("this computer have 32GB RAM and it's trashing disk! grr")

garaetjjte 7y ago

>you don't have to do a hard power cut to stop the disk from thrashing You can rescue from that situation by invoking OOM killer from SysRq magic key.