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Date:	Sat, 17 Jul 2010 17:13:34 -0400
From:	Ed Tomlinson <>
To:	Nitin Gupta <>
Cc:	"linux-mm" <>,
	"linux-kernel" <>
Subject: Re: [PATCH 0/8] zcache: page cache compression support


Would you have all this in a git tree somewhere?

Considering getting this working requires 24 patches it would really help with testing.

Ed Tomlinson

On Friday 16 July 2010 08:37:42 you 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:
> 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:
>  - Then apply this patch series; also uploaded here:
> 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
> --
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