udev/zram: Rewrite comment about swappiness

Signed-off-by: Vasiliy Stelmachenok <[email protected]>

usr/lib/udev/rules.d/30-zram.rules

@@ -1,10 +1,14 @@
 TEST!="/dev/zram0", GOTO="zram_end"
 
-# Since ZRAM stores all pages in compressed form in RAM, we should prefer
-# preempting anonymous pages more than a page (file) cache.  Preempting file
-# pages may not be desirable because a process may want to access a file at any
-# time, whereas if it is preempted, it will cause an additional read cycle from
-# the disk.
+# When used with ZRAM, it is better to prefer page out only anonymous pages,
+# because it ensures that they do not go out of memory, but will be just
+# compressed. If we do frequent flushing of file pages, that increases the
+# percentage of page cache misses, which in the long term gives additional
+# cycles to re-read the same data from disk that was previously in page cache.
+# This is the reason why it is recommended to use high values from 100 to keep
+# the page cache as hermetic as possible, because otherwise it is "expensive"
+# to read data from disk again. At the same time, uncompressing pages from ZRAM
+# is not as expensive and is usually very fast on modern CPUs.
 SYSCTL{vm.swappiness}="150"
 
 LABEL="zram_end"