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    SubjectRe: [PATCH 0/8] zcache: page cache compression support

    ----- "Nitin Gupta" <ngupta@vflare.org> wrote:

    > Frequently accessed filesystem data is stored in memory to reduce
    > access to
    > (much) slower backing disks. Under memory pressure, these pages are
    > freed and
    > when needed again, they have to be read from disks again. When
    > combined working
    > set of all running application exceeds amount of physical RAM, we get
    > extereme
    > slowdown as reading a page from disk can take time in order of
    > milliseconds.
    >
    > Memory compression increases effective memory size and allows more
    > pages to
    > stay in RAM. Since de/compressing memory pages is several orders of
    > magnitude
    > faster than disk I/O, this can provide signifant performance gains for
    > many
    > workloads. Also, with multi-cores becoming common, benefits of reduced
    > disk I/O
    > should easily outweigh the problem of increased CPU usage.
    >
    > It is implemented as a "backend" for cleancache_ops [1] which
    > provides
    > callbacks for events such as when a page is to be removed from the
    > page cache
    > and when it is required again. We use them to implement a 'second
    > chance' cache
    > for these evicted page cache pages by compressing and storing them in
    > memory
    > itself.
    >
    > We only keep pages that compress to PAGE_SIZE/2 or less. Compressed
    > chunks are
    > stored using xvmalloc memory allocator which is already being used by
    > zram
    > driver for the same purpose. Zero-filled pages are checked and no
    > memory is
    > allocated for them.
    >
    > A separate "pool" is created for each mount instance for a
    > cleancache-aware
    > filesystem. Each incoming page is identified with <pool_id, inode_no,
    > index>
    > where inode_no identifies file within the filesystem corresponding to
    > pool_id
    > and index is offset of the page within this inode. Within a pool,
    > inodes are
    > maintained in an rb-tree and each of its nodes points to a separate
    > radix-tree
    > which maintains list of pages within that inode.
    >
    > While compression reduces disk I/O, it also reduces the space
    > available for
    > normal (uncompressed) page cache. This can result in more frequent
    > page cache
    > reclaim and thus higher CPU overhead. Thus, it's important to maintain
    > good hit
    > rate for compressed cache or increased CPU overhead can nullify any
    > other
    > benefits. This requires adaptive (compressed) cache resizing and page
    > replacement policies that can maintain optimal cache size and quickly
    > reclaim
    > unused compressed chunks. This work is yet to be done. However, in the
    > current
    > state, it allows manually resizing cache size using (per-pool) sysfs
    > node
    > 'memlimit' which in turn frees any excess pages *sigh* randomly.
    >
    > Finally, it uses percpu stats and compression buffers to allow better
    > performance on multi-cores. Still, there are known bottlenecks like a
    > single
    > xvmalloc mempool per zcache pool and few others. I will work on this
    > when I
    > start with profiling.
    >
    > * Performance numbers:
    > - Tested using iozone filesystem benchmark
    > - 4 CPUs, 1G RAM
    > - Read performance gain: ~2.5X
    > - Random read performance gain: ~3X
    > - In general, performance gains for every kind of I/O
    >
    > Test details with graphs can be found here:
    > http://code.google.com/p/compcache/wiki/zcacheIOzone
    >
    > If I can get some help with testing, it would be intersting to find
    > its
    > effect in more real-life workloads. In particular, I'm intersted in
    > finding
    > out its effect in KVM virtualization case where it can potentially
    > allow
    > running more number of VMs per-host for a given amount of RAM. With
    > zcache
    > enabled, VMs can be assigned much smaller amount of memory since host
    > can now
    > hold bulk of page-cache pages, allowing VMs to maintain similar level
    > of
    > performance while a greater number of them can be hosted.
    >
    > * How to test:
    > All patches are against 2.6.35-rc5:
    >
    > - First, apply all prerequisite patches here:
    > http://compcache.googlecode.com/hg/sub-projects/zcache_base_patches
    >
    > - Then apply this patch series; also uploaded here:
    > http://compcache.googlecode.com/hg/sub-projects/zcache_patches
    >
    >
    > Nitin Gupta (8):
    > Allow sharing xvmalloc for zram and zcache
    > Basic zcache functionality
    > Create sysfs nodes and export basic statistics
    > Shrink zcache based on memlimit
    > Eliminate zero-filled pages
    > Compress pages using LZO
    > Use xvmalloc to store compressed chunks
    > Document sysfs entries
    >
    > Documentation/ABI/testing/sysfs-kernel-mm-zcache | 53 +
    > drivers/staging/Makefile | 2 +
    > drivers/staging/zram/Kconfig | 22 +
    > drivers/staging/zram/Makefile | 5 +-
    > drivers/staging/zram/xvmalloc.c | 8 +
    > drivers/staging/zram/zcache_drv.c | 1312
    > ++++++++++++++++++++++
    > drivers/staging/zram/zcache_drv.h | 90 ++
    > 7 files changed, 1491 insertions(+), 1 deletions(-)
    > create mode 100644 Documentation/ABI/testing/sysfs-kernel-mm-zcache
    > create mode 100644 drivers/staging/zram/zcache_drv.c
    > create mode 100644 drivers/staging/zram/zcache_drv.h
    By tested those patches on the top of the linus tree at this commit d0c6f6258478e1dba532bf7c28e2cd6e1047d3a4, the OOM was trigger even though there looked like still lots of swap.

    # free -m
    total used free shared buffers cached
    Mem: 852 379 473 0 3 15
    -/+ buffers/cache: 359 492
    Swap: 2015 14 2001

    # ./usemem 1024
    0: Mallocing 32 megabytes
    1: Mallocing 32 megabytes
    2: Mallocing 32 megabytes
    3: Mallocing 32 megabytes
    4: Mallocing 32 megabytes
    5: Mallocing 32 megabytes
    6: Mallocing 32 megabytes
    7: Mallocing 32 megabytes
    8: Mallocing 32 megabytes
    9: Mallocing 32 megabytes
    10: Mallocing 32 megabytes
    11: Mallocing 32 megabytes
    12: Mallocing 32 megabytes
    13: Mallocing 32 megabytes
    14: Mallocing 32 megabytes
    15: Mallocing 32 megabytes
    Connection to 192.168.122.193 closed.

    usemem invoked oom-killer: gfp_mask=0x280da, order=0, oom_adj=0
    usemem cpuset=/ mems_allowed=0
    Pid: 1829, comm: usemem Not tainted 2.6.35-rc5+ #5
    Call Trace:
    [<ffffffff814e10cb>] ? _raw_spin_unlock+0x2b/0x40
    [<ffffffff81108520>] dump_header+0x70/0x190
    [<ffffffff811086c1>] oom_kill_process+0x81/0x180
    [<ffffffff81108c08>] __out_of_memory+0x58/0xd0
    [<ffffffff81108ddc>] ? out_of_memory+0x15c/0x1f0
    [<ffffffff81108d8f>] out_of_memory+0x10f/0x1f0
    [<ffffffff8110cc7f>] __alloc_pages_nodemask+0x7af/0x7c0
    [<ffffffff81140a69>] alloc_page_vma+0x89/0x140
    [<ffffffff81125f76>] handle_mm_fault+0x6d6/0x990
    [<ffffffff814e10cb>] ? _raw_spin_unlock+0x2b/0x40
    [<ffffffff81121afd>] ? follow_page+0x19d/0x350
    [<ffffffff8112639c>] __get_user_pages+0x16c/0x480
    [<ffffffff810127c9>] ? sched_clock+0x9/0x10
    [<ffffffff811276ef>] __mlock_vma_pages_range+0xef/0x1f0
    [<ffffffff81127f01>] mlock_vma_pages_range+0x91/0xa0
    [<ffffffff8112ad57>] mmap_region+0x307/0x5b0
    [<ffffffff8112b354>] do_mmap_pgoff+0x354/0x3a0
    [<ffffffff8112b3fc>] ? sys_mmap_pgoff+0x5c/0x200
    [<ffffffff8112b41a>] sys_mmap_pgoff+0x7a/0x200
    [<ffffffff814e02f2>] ? trace_hardirqs_on_thunk+0x3a/0x3f
    [<ffffffff8100fa09>] sys_mmap+0x29/0x30
    [<ffffffff8100b032>] system_call_fastpath+0x16/0x1b
    Mem-Info:
    Node 0 DMA per-cpu:
    CPU 0: hi: 0, btch: 1 usd: 0
    CPU 1: hi: 0, btch: 1 usd: 0
    Node 0 DMA32 per-cpu:
    CPU 0: hi: 186, btch: 31 usd: 140
    CPU 1: hi: 186, btch: 31 usd: 47
    active_anon:128 inactive_anon:140 isolated_anon:0
    active_file:0 inactive_file:9 isolated_file:0
    unevictable:126855 dirty:0 writeback:125 unstable:0
    free:1996 slab_reclaimable:4445 slab_unreclaimable:23646
    mapped:923 shmem:7 pagetables:778 bounce:0
    Node 0 DMA free:4032kB min:60kB low:72kB high:88kB active_anon:0kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:11896kB isolated(anon):0kB isolated(file):0kB present:15756kB mlocked:11896kB dirty:0kB writeback:0kB mapped:0kB shmem:0kB slab_reclaimable:0kB slab_unreclaimable:0kB kernel_stack:0kB pagetables:24kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? yes
    lowmem_reserve[]: 0 994 994 994
    Node 0 DMA32 free:3952kB min:4000kB low:5000kB high:6000kB active_anon:512kB inactive_anon:560kB active_file:0kB inactive_file:36kB unevictable:495524kB isolated(anon):0kB isolated(file):0kB present:1018060kB mlocked:495524kB dirty:0kB writeback:500kB mapped:3692kB shmem:28kB slab_reclaimable:17780kB slab_unreclaimable:94584kB kernel_stack:1296kB pagetables:3088kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:1726 all_unreclaimable? yes
    lowmem_reserve[]: 0 0 0 0
    Node 0 DMA: 0*4kB 2*8kB 1*16kB 1*32kB 2*64kB 2*128kB 2*256kB 0*512kB 1*1024kB 1*2048kB 0*4096kB = 4032kB
    Node 0 DMA32: 476*4kB 0*8kB 0*16kB 0*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 1*2048kB 0*4096kB = 3952kB
    1146 total pagecache pages
    215 pages in swap cache
    Swap cache stats: add 19633, delete 19418, find 941/1333
    Free swap = 2051080kB
    Total swap = 2064380kB
    262138 pages RAM
    43914 pages reserved
    4832 pages shared
    155665 pages non-shared
    Out of memory: kill process 1727 (console-kit-dae) score 1027939 or a child
    Killed process 1727 (console-kit-dae) vsz:4111756kB, anon-rss:0kB, file-rss:600kB
    console-kit-dae invoked oom-killer: gfp_mask=0xd0, order=0, oom_adj=0
    console-kit-dae cpuset=/ mems_allowed=0
    Pid: 1752, comm: console-kit-dae Not tainted 2.6.35-rc5+ #5
    Call Trace:
    [<ffffffff814e10cb>] ? _raw_spin_unlock+0x2b/0x40
    [<ffffffff81108520>] dump_header+0x70/0x190
    [<ffffffff811086c1>] oom_kill_process+0x81/0x180
    [<ffffffff81108c08>] __out_of_memory+0x58/0xd0
    [<ffffffff81108ddc>] ? out_of_memory+0x15c/0x1f0
    [<ffffffff81108d8f>] out_of_memory+0x10f/0x1f0
    [<ffffffff8110cc7f>] __alloc_pages_nodemask+0x7af/0x7c0
    [<ffffffff8114522e>] kmem_getpages+0x6e/0x180
    [<ffffffff81147d79>] fallback_alloc+0x1c9/0x2b0
    [<ffffffff81147602>] ? cache_grow+0x4b2/0x520
    [<ffffffff81147a5b>] ____cache_alloc_node+0xab/0x200
    [<ffffffff810d55d5>] ? taskstats_exit+0x305/0x3b0
    [<ffffffff8114862b>] kmem_cache_alloc+0x1fb/0x290
    [<ffffffff810d55d5>] taskstats_exit+0x305/0x3b0
    [<ffffffff81063a4b>] do_exit+0x12b/0x890
    [<ffffffff810924fd>] ? trace_hardirqs_off+0xd/0x10
    [<ffffffff8108641f>] ? cpu_clock+0x6f/0x80
    [<ffffffff81095cbd>] ? lock_release_holdtime+0x3d/0x190
    [<ffffffff814e1010>] ? _raw_spin_unlock_irq+0x30/0x40
    [<ffffffff8106420e>] do_group_exit+0x5e/0xd0
    [<ffffffff81075b54>] get_signal_to_deliver+0x2d4/0x490
    [<ffffffff811ea6ad>] ? inode_has_perm+0x7d/0xf0
    [<ffffffff8100a2e5>] do_signal+0x75/0x7b0
    [<ffffffff81169d2d>] ? vfs_ioctl+0x3d/0xf0
    [<ffffffff8116a394>] ? do_vfs_ioctl+0x84/0x570
    [<ffffffff8100aa85>] do_notify_resume+0x65/0x80
    [<ffffffff814e02f2>] ? trace_hardirqs_on_thunk+0x3a/0x3f
    [<ffffffff8100b381>] int_signal+0x12/0x17
    Mem-Info:
    Node 0 DMA per-cpu:
    CPU 0: hi: 0, btch: 1 usd: 0
    CPU 1: hi: 0, btch: 1 usd: 0
    Node 0 DMA32 per-cpu:
    CPU 0: hi: 186, btch: 31 usd: 151
    CPU 1: hi: 186, btch: 31 usd: 61
    active_anon:128 inactive_anon:165 isolated_anon:0
    active_file:0 inactive_file:9 isolated_file:0
    unevictable:126855 dirty:0 writeback:25 unstable:0
    free:1965 slab_reclaimable:4445 slab_unreclaimable:23646
    mapped:923 shmem:7 pagetables:778 bounce:0
    Node 0 DMA free:4032kB min:60kB low:72kB high:88kB active_anon:0kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:11896kB isolated(anon):0kB isolated(file):0kB present:15756kB mlocked:11896kB dirty:0kB writeback:0kB mapped:0kB shmem:0kB slab_reclaimable:0kB slab_unreclaimable:0kB kernel_stack:0kB pagetables:24kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? yes
    lowmem_reserve[]: 0 994 994 994
    Node 0 DMA32 free:3828kB min:4000kB low:5000kB high:6000kB active_anon:512kB inactive_anon:660kB active_file:0kB inactive_file:36kB unevictable:495524kB isolated(anon):0kB isolated(file):0kB present:1018060kB mlocked:495524kB dirty:0kB writeback:100kB mapped:3692kB shmem:28kB slab_reclaimable:17780kB slab_unreclaimable:94584kB kernel_stack:1296kB pagetables:3088kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:1726 all_unreclaimable? yes
    lowmem_reserve[]: 0 0 0 0
    Node 0 DMA: 0*4kB 2*8kB 1*16kB 1*32kB 2*64kB 2*128kB 2*256kB 0*512kB 1*1024kB 1*2048kB 0*4096kB = 4032kB
    Node 0 DMA32: 445*4kB 0*8kB 0*16kB 0*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 1*2048kB 0*4096kB = 3828kB
    1146 total pagecache pages
    230 pages in swap cache
    Swap cache stats: add 19649, delete 19419, find 942/1336
    Free swap = 2051084kB
    Total swap = 2064380kB
    262138 pages RAM
    43914 pages reserved
    4818 pages shared
    155685 pages non-shared
    Out of memory: kill process 1806 (sshd) score 9474 or a child
    Killed process 1810 (bash) vsz:108384kB, anon-rss:0kB, file-rss:656kB
    console-kit-dae invoked oom-killer: gfp_mask=0xd0, order=0, oom_adj=0
    console-kit-dae cpuset=/ mems_allowed=0
    Pid: 1752, comm: console-kit-dae Not tainted 2.6.35-rc5+ #5
    Call Trace:
    [<ffffffff814e10cb>] ? _raw_spin_unlock+0x2b/0x40
    [<ffffffff81108520>] dump_header+0x70/0x190
    [<ffffffff811086c1>] oom_kill_process+0x81/0x180
    [<ffffffff81108c08>] __out_of_memory+0x58/0xd0
    [<ffffffff81108ddc>] ? out_of_memory+0x15c/0x1f0
    [<ffffffff81108d8f>] out_of_memory+0x10f/0x1f0
    [<ffffffff8110cc7f>] __alloc_pages_nodemask+0x7af/0x7c0
    [<ffffffff8114522e>] kmem_getpages+0x6e/0x180
    [<ffffffff81147d79>] fallback_alloc+0x1c9/0x2b0
    [<ffffffff81147602>] ? cache_grow+0x4b2/0x520
    [<ffffffff81147a5b>] ____cache_alloc_node+0xab/0x200
    [<ffffffff810d55d5>] ? taskstats_exit+0x305/0x3b0
    [<ffffffff8114862b>] kmem_cache_alloc+0x1fb/0x290
    [<ffffffff810d55d5>] taskstats_exit+0x305/0x3b0
    [<ffffffff81063a4b>] do_exit+0x12b/0x890
    [<ffffffff810924fd>] ? trace_hardirqs_off+0xd/0x10
    [<ffffffff8108641f>] ? cpu_clock+0x6f/0x80
    [<ffffffff81095cbd>] ? lock_release_holdtime+0x3d/0x190
    [<ffffffff814e1010>] ? _raw_spin_unlock_irq+0x30/0x40
    [<ffffffff8106420e>] do_group_exit+0x5e/0xd0
    [<ffffffff81075b54>] get_signal_to_deliver+0x2d4/0x490
    [<ffffffff811ea6ad>] ? inode_has_perm+0x7d/0xf0
    [<ffffffff8100a2e5>] do_signal+0x75/0x7b0
    [<ffffffff81169d2d>] ? vfs_ioctl+0x3d/0xf0
    [<ffffffff8116a394>] ? do_vfs_ioctl+0x84/0x570
    [<ffffffff8100aa85>] do_notify_resume+0x65/0x80
    [<ffffffff814e02f2>] ? trace_hardirqs_on_thunk+0x3a/0x3f
    [<ffffffff8100b381>] int_signal+0x12/0x17
    Mem-Info:
    Node 0 DMA per-cpu:
    CPU 0: hi: 0, btch: 1 usd: 0
    CPU 1: hi: 0, btch: 1 usd: 0
    Node 0 DMA32 per-cpu:
    CPU 0: hi: 186, btch: 31 usd: 119
    CPU 1: hi: 186, btch: 31 usd: 73
    active_anon:50 inactive_anon:175 isolated_anon:0
    active_file:0 inactive_file:9 isolated_file:0
    unevictable:126855 dirty:0 writeback:25 unstable:0
    free:1996 slab_reclaimable:4445 slab_unreclaimable:23663
    mapped:923 shmem:7 pagetables:778 bounce:0
    Node 0 DMA free:4032kB min:60kB low:72kB high:88kB active_anon:0kB inactive_anon:0kB active_file:0kB inactive_file:0kB unevictable:11896kB isolated(anon):0kB isolated(file):0kB present:15756kB mlocked:11896kB dirty:0kB writeback:0kB mapped:0kB shmem:0kB slab_reclaimable:0kB slab_unreclaimable:0kB kernel_stack:0kB pagetables:24kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? yes
    lowmem_reserve[]: 0 994 994 994
    Node 0 DMA32 free:3952kB min:4000kB low:5000kB high:6000kB active_anon:200kB inactive_anon:700kB active_file:0kB inactive_file:36kB unevictable:495524kB isolated(anon):0kB isolated(file):0kB present:1018060kB mlocked:495524kB dirty:0kB writeback:100kB mapped:3692kB shmem:28kB slab_reclaimable:17780kB slab_unreclaimable:94652kB kernel_stack:1296kB pagetables:3088kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:1536 all_unreclaimable? yes
    lowmem_reserve[]: 0 0 0 0
    Node 0 DMA: 0*4kB 2*8kB 1*16kB 1*32kB 2*64kB 2*128kB 2*256kB 0*512kB 1*1024kB 1*2048kB 0*4096kB = 4032kB
    Node 0 DMA32: 470*4kB 3*8kB 0*16kB 0*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 1*2048kB 0*4096kB = 3952kB
    1146 total pagecache pages
    221 pages in swap cache
    Swap cache stats: add 19848, delete 19627, find 970/1386
    Free swap = 2051428kB
    Total swap = 2064380kB
    262138 pages RAM
    43914 pages reserved
    4669 pages shared
    155659 pages non-shared
    Out of memory: kill process 1829 (usemem) score 8253 or a child
    Killed process 1829 (usemem) vsz:528224kB, anon-rss:502468kB, file-rss:376kB

    # cat usemem.c
    # cat usemem.c
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/mman.h>
    #define CHUNKS 32

    int
    main(int argc, char *argv[])
    {
    mlockall(MCL_FUTURE);

    unsigned long mb;
    char *buf[CHUNKS];
    int i;

    if (argc < 2) {
    fprintf(stderr, "usage: usemem megabytes\n");
    exit(1);
    }
    mb = strtoul(argv[1], NULL, 0);

    for (i = 0; i < CHUNKS; i++) {
    fprintf(stderr, "%d: Mallocing %lu megabytes\n", i, mb/CHUNKS);
    buf[i] = (char *)malloc(mb/CHUNKS * 1024L * 1024L);
    if (!buf[i]) {
    fprintf(stderr, "malloc failure\n");
    exit(1);
    }
    }

    for (i = 0; i < CHUNKS; i++) {
    fprintf(stderr, "%d: Zeroing %lu megabytes at %p\n",
    i, mb/CHUNKS, buf[i]);
    memset(buf[i], 0, mb/CHUNKS * 1024L * 1024L);
    }


    exit(0);
    }


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