> From: Ric Mason [mailto:[email protected]] > Sent: Saturday, February 16, 2013 11:19 PM > To: Minchan Kim > Cc: Greg Kroah-Hartman; [email protected]; [email protected]; > Andrew Morton; Seth > Jennings; Nitin Gupta; Dan Magenheimer; Konrad Rzeszutek Wilk > Subject: Re: [PATCH v2] zsmalloc: Add Kconfig for enabling PTE method > > On 02/06/2013 10:17 AM, Minchan Kim wrote: > > Zsmalloc has two methods 1) copy-based and 2) pte-based to access > > allocations that span two pages. You can see history why we supported > > two approach from [1]. > > > > In summary, copy-based method is 3 times fater in x86 while pte-based > > is 6 times faster in ARM. > > Why in some arches copy-based method is better and in the other arches > pte-based is better? What's the root reason?
Minchan, if you post another version, I think these precise numbers (of "times faster") should be removed. The speed is very data dependent, because the copy-based method is copying a zpage which may vary widely in size from ~100 bytes to nearly PAGE_SIZE bytes, a factor of 40x or more. Please at least say "up to 3 times" or "approximately 3x faster for an average compressed page". Ric, the copy-based method does an extra copy of N bytes (where N is the compressed size of a page). The pte-based method requires extra TLB actions. The relative speed of TLB operations vs copying is very architecture-dependent. It is also probably dependent on the specific implementation of the architecture (i.e x86 sandybridge is likely very different than x86 nehalem) and, as noted above, dependent on N which is unpredictable. So it makes sense to have both choices, but it's not at all clear how to select which one to use! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
