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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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