Am Thu, 25 May 2017 11:46:45 -0700 schrieb Ian Zimmerman <i...@primate.net>:
> On 2017-05-24 19:05, Kai Krakow wrote: > > > To get in line with Rich Freeman: I didn't want to imply that zswap > > only works with swap, neither that tmpfs only works with swap. Both > > work without. But if you want to put some serious amount of data > > into tmpfs, you need swap as a backing device sooner or later. > > Looking at zswap, I have several questions > (even after reading linux/Documentation/vm/zswap.txt). > > 1. How does it know which swap device to use as backing store, if > any? Clearly at boot time no swap configuration exists, even if > initrd/initramfs is used, which here it is not. So when the kernel > sees zswap.enable=1 in the command line, what happens? You simply don't assign a swap device to zswap. It's transparently inserted into the swapping chain of the kernel. Thus pages are first compressed, and later swapped out by normal kernel processing. > 2. The doc says it can be turned on at runtime by means of > /sys/module/zswap/parameters/enabled. But kconfig doesn't make it > possible to build the support as a module, only built-in, and so it is > not surprising that this path doesn't exist. I wonder why this doesn't exist. All my builtin modules have their parameters in /sys/module: # lsmod | fgrep zswap | wc -l 0 # ls -ald /sys/module/zswap drwxr-xr-x 3 root root 0 26. Mai 07:54 /sys/module/zswap > 3. It seems to require zbud to also be turned on, but this is not > enforced by kconfig. Is this a bug or what? No idea, I enabled it... > 4. Quoting: > > Zswap seeks to be simple in its policies. Sysfs attributes allow > for one user controlled policy: > * max_pool_percent - The maximum percentage of memory that the > compressed pool can occupy. > > Does this mean this is another (hypothetical) node in > /sys/module/zswap/parameters/ ? grep ^ /sys/module/zswap/parameters/* /sys/module/zswap/parameters/compressor:lzo /sys/module/zswap/parameters/enabled:Y /sys/module/zswap/parameters/max_pool_percent:20 /sys/module/zswap/parameters/zpool:zbud This also implies that zbud it required for zswap to even operate. If you didn't include it, it may be the reason why zswap is missing in /sys/module. -- Regards, Kai Replies to list-only preferred.
