Linux – Swap Out in Bigger Chunks

linuxswap

My swapdisk can write 100 MB/s. When the system swaps out it only writes 8-20 MB/s.

iostat says the device is 100% active, so I have the feeling that Linux is seeking on the drive or swapping out small chunks.

I could explain this if there were swap-ins at the same time, but there are no swap-ins and no other disk I/O.

Is it possible for me to tell Linux to swap out in bigger contiguous chunks, say, determining the oldest 10 MB pages and swap them out in one chunk?

A bit like saying the page frame size of swap is not 4K but 10M.

For my system I think an algorithm along the lines of this would be ideal:

dirty_pct = dirty pages / all RAM
if dirty_pct > 50%:
   # Half of memory is dirty, slowly start swapping out
   if busy_time(swapdevice) < dirty_pct:
       # if RAM is 60% dirty, start swapping if disk is less than 60% busy
       # if RAM is 90% dirty, start swapping if disk is less than 90% busy
       identify the next 10 MB that would be swapped if dirty_pct had been 100%
       save the 10 MB to swap as a single chunk
       mark the pages as clean

This way my system would start swapping out at 50% dirty and it would not affect performance, because it would do so on a drive that was sitting idle anyway. Maybe the swapped data will never be used, and then we wasted some IO that was sitting idle anyway.

$ uname -a
Linux r815 4.15.0-99-generic #100-Ubuntu SMP Wed Apr 22 20:32:56 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

Best Answer

Not. It isn't possible.

Linux Kernel decide through different algorithms about RAM, Swap Spaces, Last Swap Location Used, Disk Cache Responsive, Swapiness and so other things when and how to use Swap memory allocation.

The system can't just wait to have "10MB> page collection" because memory managing is on-demand to avoid system freezes and process killing.

However, some bugs related to RAM and Swapping management have been reported during the years due to really bad performance when the system starts to swap.

I can say you that Disk Usage is improvable using zswap, because the biggest problem here is the exponential multiplication of pages when arriving swap (that's because memory pages linking).

zswap compress that multiplication, making less disk reads/writes and can have a prominent impact on your CPU usage.

zswap is kernel-managed and you have a lot of useful low-level configurations to adapt it to your system needs.

https://www.maketecheasier.com/use-zswap-improve-old-linux-performance/

Related Solutions

Linux – How to tweak the kernel for total swap out

You can set the value of /proc/sys/vm/swappiness to control the ratio of segments of data swapped to the segments of data kept in memory. A value of 0 completely avoids swapping at all costs.

This can be done using either:

echo 0 > /proc/sys/vm/swappiness
sysctl -w vm.swappiness=0
Storing that setting in /etc/sysctl.conf

Generally, using just a little swap is not a bad thing. Free memory can be used for caching data read from disk, and the system can plan ahead for a sudden need of lots of memory by an application.

When too many programs are swapped however, there is a lot of disk related activity during every program switch which really makes everything slow down. Before something can be used, it needs to be loaded back into memory.

Disks reads are horribly slow compared to memory access, as it takes significantly longer for the data to arrive. The system has to schedule the read between the other read/write requests, the drive starts making attempts to find the right cylinder, and finally starts slowly delivering data.

Hence, I think your logic is flawed. Generally, you want to keep programs running in memory, while still keeping enough room for sudden growth. Do not use the swap too often to "write things to disk", because it is neither a backup nor a performance improvement.

Older computers contained less memory and suffered from swapping problems as a result. When many programs were open at once, the system would slow down and you could hear the disk reading and writing in order to the swap file.

Linux – the appropriate value of vm.swappiness when using zram

Short answer:vm.swappiness=100is appropriate value for zram(At least on Debian Stretch with Linux 4.9 , I believe that is best value )

I already test vm.swappiness=100 for me.

I think you can do some simple test to sure which value is best for you.

Also I made another simple program for test this question. x On my machine a very low vm.swappiness value(such as vm.swappiness=1 ) will cause obvious responsiveness problem.

About SwapCached in /proc/meminfo:

First,try vm.page-cluster=0,this maybe can reduce some useless SwapCached from swap-in.

SwapCached can speed up zram same as non-zram swap device

SwapCached is can reuse(free) when necessary:

./linux-4.9/mm$ grep -rn delete_from_swap_cache
memory-failure.c:715:   delete_from_swap_cache(p);
shmem.c:1115:       delete_from_swap_cache(*pagep);
shmem.c:1645:            * unaccounting, now delete_from_swap_cache() will do
shmem.c:1652:               delete_from_swap_cache(page);
shmem.c:1668:       delete_from_swap_cache(page);
vmscan.c:673:       __delete_from_swap_cache(page);
swap_state.c:137:void __delete_from_swap_cache(struct page *page)
swap_state.c:218:void delete_from_swap_cache(struct page *page)
swap_state.c:227:   __delete_from_swap_cache(page);
swapfile.c:947:         delete_from_swap_cache(page);
swapfile.c:987: delete_from_swap_cache(page);
swapfile.c:1023:            delete_from_swap_cache(page);
swapfile.c:1571:            delete_from_swap_cache(page);
./linux-4.9/mm$

Best Answer

Related Solutions

Linux – How to tweak the kernel for total swap out

Linux – the appropriate value of vm.swappiness when using zram

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