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

Pekka Enberg
2010-07-18T07:50:14Z

Nitin Gupta 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.

So why would someone want to use zram if they have transparent page
cache compression with zcache? That is, why is this not a replacement
for zram?

Pekka