On Fri, Jun 05, 2020 at 01:50:29AM -0600, Chris Murphy wrote: > On Fri, Jun 5, 2020 at 12:33 AM Milan Crha <mcrha@xxxxxxxxxx> wrote: > > > > On Thu, 2020-06-04 at 16:30 -0400, Ben Cotton wrote: > > > ... The memory used is not preallocated. It's > > > dynamically allocated and deallocated, on demand. ... > > > > > > The system will use RAM normally up until it's full, and then start > > > paging out to swap-on-zram, same as a conventional swap-on-drive.... > > > > Hi, > > I confess I've absolutely no idea about this, I mean how that works in > > practice, but when you tell me "we do not allocate on start, we > > allocate on demand, when the memory is full", then, I hope a logic > > question is, where do you allocate, when the memory is already full? If > > there's some threshold, then it's quite the same as preallocate the > > memory. > > Yes, it's an oversimplification. There isn't a case when the memory is > truly completely full. The kernel starts to swap before that, and it > can move things around from buffers, cached, and even do reclaim, in > order to start allocating memory to the zram device. And at that point > the compression permits a greater rate of freed memory due to > eviction, than loss due to the allocation to the zram device. > > This is taken just now from a laptop with 2+ days uptime > > ]$ free -m > total used free shared buff/cache available > Mem: 7845 3292 699 532 3852 3714 > Swap: 3921 192 3729 > $ swapon > NAME TYPE SIZE USED PRIO > /dev/zram0 partition 3.8G 193M -2 > $ zramctl > NAME ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT > /dev/zram0 lzo-rle 3.9G 187.3M 50M 52.7M 4 [SWAP] > $ > > This is a small amount of swap in use right now. In this case, ~190M > has been paged out to swap, and the zram device has compressed that to > ~50M. That's pretty good, so it suggests highly compressible data. The > savings is ~140M. That 140M can be used to avoid reclaim of file pages > (programs can stay in RAM instead of being pushed out and read back in > later) or for more active user data, etc. Anything really. I'm unclear: that ~50M is still in RAM? Or it's compressed on a disk somewhere? Also does the swap partition on disk contain compressed pages, or uncompressed pages, or a mix of both? Also what is the compression algorithm? zlib or zstd or something else? The description (I think copied from the kernel documentation) was really unclear about how this works. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html _______________________________________________ devel mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@xxxxxxxxxxxxxxxxxxxxxxx
