[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: [Xen-devel] Re: [RFC PATCH 0/4] (Take 2): transcendent memory ("tmem") for Linux
Hi Anthony -- Thanks for the comments. > I have trouble mapping this to a VMM capable of overcommit > without just coming back to CMM2. > > In CMM2 parlance, ephemeral tmem pools is just normal kernel memory > marked in the volatile state, no? They are similar in concept, but a volatile-marked kernel page is still a kernel page, can be changed by a kernel (or user) store instruction, and counts as part of the memory used by the VM. An ephemeral tmem page cannot be directly written by a kernel (or user) store, can only be read via a "get" (which may or may not succeed), and doesn't count against the memory used by the VM (even though it likely contains -- for awhile -- data useful to the VM). > It seems to me that an architecture built around hinting > would be more > robust than having to use separate memory pools for this type > of memory > (especially since you are requiring a copy to/from the pool). Depends on what you mean by robust, I suppose. Once you understand the basics of tmem, it is very simple and this is borne out in the low invasiveness of the Linux patch. Simplicity is another form of robustness. > For instance, you can mark data DMA'd from disk (perhaps by > read-ahead) > as volatile without ever bringing it into the CPU cache. > With tmem, if > you wanted to use a tmem pool for all of the page cache, you'd likely > suffer significant overhead due to copying. The copy may be expensive on an older machine, but on newer machines copying a page is relatively inexpensive. On a reasonable multi-VM-kernbench-like benchmark I'll be presenting at Linux Symposium next week, the overhead is on the order of 0.01% for a fairly significant savings in IOs. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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