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Subjectzswap: How to determine whether it is compressing swap pages?
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Hi Seth, hi everyone,

Yesterday I build 3.11-rc1 with CONFIG_ZSWAP and wanted to test it.

I added zswap.enabled=1 and get:

martin@merkaba:~> dmesg | grep zswap
[ 0.000000] Command line: BOOT_IMAGE=/vmlinuz-3.11.0-rc1-tp520+
root=/dev/mapper/merkaba-debian ro rootflags=subvol=root init=/bin/systemd
cgroup_enable=memory threadirqs i915.i915_enable_rc6=7 zcache zswap.enabled=1
[ 0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-3.11.0-rc1-tp520+
root=/dev/mapper/merkaba-debian ro rootflags=subvol=root init=/bin/systemd
cgroup_enable=memory threadirqs i915.i915_enable_rc6=7 zcache zswap.enabled=1
[ 1.452443] zswap: loading zswap
[ 1.452465] zswap: using lzo compressor

I did a stress -m 1 --vm-keep --vm-bytes 4G on this 8 GB ThinkPad T520 in
order to allocate some swap.

Still I think zswap didn´t do anything:

merkaba:/sys/kernel/debug/zswap> grep . *
duplicate_entry:0
pool_limit_hit:0
pool_pages:0
reject_alloc_fail:0
reject_compress_poor:0
reject_kmemcache_fail:0
reject_reclaim_fail:0
stored_pages:0
written_back_pages:0


However:

merkaba:/sys/kernel/slab/zswap_entry> grep . *
aliases:9
align:8
grep: alloc_calls: Die angeforderte Funktion ist nicht implementiert
cache_dma:0
cpu_partial:0
cpu_slabs:4 N0=4
destroy_by_rcu:0
grep: free_calls: Die angeforderte Funktion ist nicht implementiert
hwcache_align:0
min_partial:5
objects:2550 N0=2550
object_size:48
objects_partial:0
objs_per_slab:85
order:0
partial:0
poison:0
reclaim_account:0
red_zone:0
remote_node_defrag_ratio:100
reserved:0
sanity_checks:0
slabs:30 N0=30
slabs_cpu_partial:0(0)
slab_size:48
store_user:0
total_objects:2550 N0=2550
trace:0

It has some objects it seems.


How do I know whether zswap actually does something?

Will zswap work even with zcache enabled? As I understand zcache compresses
swap device pages on the block device level in addition to compressing read
cache pages of usual filesystems. Which one takes precedence, zcache or zswap?
Can I disable zcache for swap device?



Here is dmesg for zcache:

martin@merkaba:~> dmesg | grep zcache
[ 0.000000] Command line: BOOT_IMAGE=/vmlinuz-3.11.0-rc1-tp520+
root=/dev/mapper/merkaba-debian ro rootflags=subvol=root init=/bin/systemd
cgroup_enable=memory threadirqs i915.i915_enable_rc6=7 zcache zswap.enabled=1
[ 0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-3.11.0-rc1-tp520+
root=/dev/mapper/merkaba-debian ro rootflags=subvol=root init=/bin/systemd
cgroup_enable=memory threadirqs i915.i915_enable_rc6=7 zcache zswap.enabled=1
[ 1.453531] zcache: using lzo compressor
[ 1.453634] zcache: cleancache enabled using kernel transcendent memory and
compression buddies
[ 1.453679] zcache: frontswap enabled using kernel transcendent memory and
compression buddies
[ 1.453722] zcache: frontswap_ops overridden
[ 5.358288] zcache: created ephemeral local tmem pool, id=0
[ 8.155684] zcache: created persistent local tmem pool, id=1
[ 8.331680] zcache: created ephemeral local tmem pool, id=2
[ 8.593235] zcache: created ephemeral local tmem pool, id=3
[ 8.743330] zcache: created ephemeral local tmem pool, id=4

Thanks,
--
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA B82F 991B EAAC A599 84C7
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