4 ms·

>While this seems an easy relationship to satisfy, more-complex models based on real-world performance parameters and access patterns indicate that it is, in re

by mrob 7y ago

>While this seems an easy relationship to satisfy, more-complex models based on real-world performance parameters and access patterns indicate that it is, in reality, very difficult to obtain a performance boost with this method. Now that improvements in CPU speed have outpaced improvements in storage speed, it's easier to benefit from compressed swap in ram. Linux includes "zswap"[0], which in my experience works well for postponing the severe performance degradation you otherwise get with heavy memory pressure on general desktop use. If you think you want to set "vm.swappiness=0" (which I ran for several years), you probably want to enable zswap instead. If you're running Debian, you can set it up by installing "zram-tools" (which can be configured with /etc/default/zramswap if you're not happy with the default). [0] https://en.wikipedia.org/wiki/Zswap https://en.wikipedia.org/wiki/Zswap

zamadatix 7y ago
Storage speed has really outpaced CPU speed for the decade zswap has been out. It still has its place but if you have modern flash storage it may well be a throughput drop and power usage increase to enable zswap (particularly with the default algorithms). Latency for pulling single pages has never really been limited by CPU performance, that's always been faster and will probably never change due to physics.
- war1025 7y ago
  As someone running older hardware with a spinning disk drive, the comment was useful and something I might consider looking into. At work I think I have a 2012-era ssd drive, that while speedy is probably pretty far behind modern drives. When things start thrashing in swap it's a bad time all around.
- newnewpdro 7y ago
  It's not like you really want to use SSD for frequently used swap though, it's just going cause unnecessary wear.
  - hinkley 7y ago
    both are probably true.
  - unionpivo 7y ago
    Stick of cheapest 1tb nvme is around 115 EUR (qlc) where I live, so even if you have to change it every 2 years it still worth it. And i measured disk usage on my desktops (home, job) and laptop, and my main dev "server" (xen, with bunch of vms). Surprisingly my desktop had most writes to disk, but even then it would take me 6 years on the cheapest (qlc, kingston 1tb) before wearing it out So i don't think that is a big issue, especially with how much you can improve your life, with faster swap.
- StillBored 7y ago
  Right, compressed ram has been used heavily on android where the secondary storage has been terribly slow (emmc anyone?). That's probably shifted as everyone moves to PCIe based storage solutions, particularly on the low-mid range phones that don't have super high performance cores.
seriesf 7y ago
squashfs is another linux feature that boils down to compressed swap.
- irth 7y ago
  it's a read only compressed file system so I am not so sure about that
  - seriesf 7y ago
    It is, though. Under memory pressure the kernel will drop any page that's backed by a file, and then page it back in as needed. "swap" is just making _all_ memory be backed by files.
    - klodolph 7y ago
      Squashfs is read-only, it can’t be used for swap.
ygra 7y ago
Windows also compresses pages to alleviate memory pressure before resorting to swap nowadays. Probably the same reason as with Android, as on lower-end hardware, especially with hard drives, compressing/decompressing isn't really noticeable, while hitting the disk is.
- pmjordan 7y ago
  macOS and iOS do this too. iOS doesn't even use any traditional swap and gets away with it by putting the onus on developers to make sure their apps never go OOM when in the foreground and to deal seamlessly with apps being OOM killed when in the background.
cookiecaper 7y ago
zswap and zram are actually different systems. zram is a compressed ramdisk that acts like a block device and can host any filesystem (including swapfs). zswap (nee compcache), on the other hand, is a region of memory that tries to store pages compressed instead of swapping them out -- basically a last resort before swapping. zswapped pages are not considered swap, so you have to check sysfs to see how much it's getting used. Many default configurations are pretty aggressive, allowing zswap to use up to 20% of physical memory before finally resorting to the system's configured swap space.

by mrob 7y ago

zamadatix 7y ago

Storage speed has really outpaced CPU speed for the decade zswap has been out. It still has its place but if you have modern flash storage it may well be a throughput drop and power usage increase to enable zswap (particularly with the default algorithms). Latency for pulling single pages has never really been limited by CPU performance, that's always been faster and will probably never change due to physics.

war1025 7y ago
As someone running older hardware with a spinning disk drive, the comment was useful and something I might consider looking into. At work I think I have a 2012-era ssd drive, that while speedy is probably pretty far behind modern drives. When things start thrashing in swap it's a bad time all around.
newnewpdro 7y ago
It's not like you really want to use SSD for frequently used swap though, it's just going cause unnecessary wear.
- hinkley 7y ago
  both are probably true.
- unionpivo 7y ago
  Stick of cheapest 1tb nvme is around 115 EUR (qlc) where I live, so even if you have to change it every 2 years it still worth it. And i measured disk usage on my desktops (home, job) and laptop, and my main dev "server" (xen, with bunch of vms). Surprisingly my desktop had most writes to disk, but even then it would take me 6 years on the cheapest (qlc, kingston 1tb) before wearing it out So i don't think that is a big issue, especially with how much you can improve your life, with faster swap.
StillBored 7y ago
Right, compressed ram has been used heavily on android where the secondary storage has been terribly slow (emmc anyone?). That's probably shifted as everyone moves to PCIe based storage solutions, particularly on the low-mid range phones that don't have super high performance cores.

seriesf 7y ago

squashfs is another linux feature that boils down to compressed swap.

irth 7y ago
it's a read only compressed file system so I am not so sure about that
- seriesf 7y ago
  It is, though. Under memory pressure the kernel will drop any page that's backed by a file, and then page it back in as needed. "swap" is just making _all_ memory be backed by files.
  - klodolph 7y ago
    Squashfs is read-only, it can’t be used for swap.

ygra 7y ago

Windows also compresses pages to alleviate memory pressure before resorting to swap nowadays. Probably the same reason as with Android, as on lower-end hardware, especially with hard drives, compressing/decompressing isn't really noticeable, while hitting the disk is.

pmjordan 7y ago
macOS and iOS do this too. iOS doesn't even use any traditional swap and gets away with it by putting the onus on developers to make sure their apps never go OOM when in the foreground and to deal seamlessly with apps being OOM killed when in the background.

cookiecaper 7y ago

zswap and zram are actually different systems. zram is a compressed ramdisk that acts like a block device and can host any filesystem (including swapfs). zswap (nee compcache), on the other hand, is a region of memory that tries to store pages compressed instead of swapping them out -- basically a last resort before swapping. zswapped pages are not considered swap, so you have to check sysfs to see how much it's getting used. Many default configurations are pretty aggressive, allowing zswap to use up to 20% of physical memory before finally resorting to the system's configured swap space.