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Date: Sun, 18 Jul 2010 07:53:28 +0530 From: Nitin Gupta <ngupta@...are.org> To: Ed Tomlinson <edt@....ca> CC: linux-mm <linux-mm@...ck.org>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 0/8] zcache: page cache compression support Hi Ed, On 07/18/2010 02:43 AM, Ed Tomlinson wrote: > > Would you have all this in a git tree somewhere? > > Considering getting this working requires 24 patches it would really help with testing. > Unfortunately, git tree for this is not hosted anywhere. Anyways, I just uploaded monolithic zcache patch containing all its dependencies: http://compcache.googlecode.com/hg/sub-projects/mainline/zcache_v1_2.6.35-rc5.patch It applies on top of 2.6.35-rc5 Thanks for trying it out. Nitin > 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: >> 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 >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >> the body of a message to majordomo@...r.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/ >> >> > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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