On 07/18/2010 01:20 PM, Pekka Enberg wrote: > 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. >> <snip> > > 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? > zcache complements zram; it's not a replacement: - zram compresses anonymous pages while zcache is for page cache compression. So, workload which depends heavily on "heap memory" usage will tend to prefer zram and those which are I/O intensive will prefer zcache. Though I have not yet experimented much, most workloads may want to have a mix of them. - zram is not just for swap. /dev/zram<id> are generic in-memory compressed block devices which can be used for, say, /tmp, /var/... etc. temporary storage. - /dev/zram<id> being a generic block devices, can be used as raw disk in other OSes also (using virtualization): For example: http://www.vflare.org/2010/05/compressed-ram-disk-for-windows-virtual.html Thanks, Nitin -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>