On 10/16/2012 11:27 AM, Ezequiel Garcia wrote: > On Tue, Oct 16, 2012 at 3:07 PM, Tim Bird <tim.bird@xxxxxxxxxxx> wrote: >> On 10/16/2012 05:56 AM, Eric Dumazet wrote: >>> On Tue, 2012-10-16 at 09:35 -0300, Ezequiel Garcia wrote: >>> >>>> Now, returning to the fragmentation. The problem with SLAB is that >>>> its smaller cache available for kmalloced objects is 32 bytes; >>>> while SLUB allows 8, 16, 24 ... >>>> >>>> Perhaps adding smaller caches to SLAB might make sense? >>>> Is there any strong reason for NOT doing this? >>> >>> I would remove small kmalloc-XX caches, as sharing a cache line >>> is sometime dangerous for performance, because of false sharing. >>> >>> They make sense only for very small hosts. >> >> That's interesting... >> >> It would be good to measure the performance/size tradeoff here. >> I'm interested in very small systems, and it might be worth >> the tradeoff, depending on how bad the performance is. Maybe >> a new config option would be useful (I can hear the groans now... :-) >> >> Ezequiel - do you have any measurements of how much memory >> is wasted by 32-byte kmalloc allocations for smaller objects, >> in the tests you've been doing? > > Yes, we have some numbers: > > http://elinux.org/Kernel_dynamic_memory_analysis#Kmalloc_objects > > Are they too informal? I can add some details... > They've been measured on a **very** minimal setup, almost every option > is stripped out, except from initramfs, sysfs, and trace. > > On this scenario, strings allocated for file names and directories > created by sysfs > are quite noticeable, being 4-16 bytes, and produce a lot of fragmentation from > that 32 byte cache at SLAB. The detail I'm interested in is the amount of wastage for a "common" workload, for each of the SLxB systems. Are we talking a few K, or 10's or 100's of K? It sounds like it's all from short strings. Are there other things using the 32-byte kmalloc cache, that waste a lot of memory (in aggregate) as well? Does your tool indicate a specific callsite (or small set of callsites) where these small allocations are made? It sounds like it's in the filesystem and would be content-driven (by the length of filenames)? This might be an issue particularly for cameras, where all the generated filenames are 8.3 (and will be for the foreseeable future) > Is an option to enable small caches on SLUB and SLAB worth it? I'll have to do some measurements to see. I'm guessing the option itself would be pretty trivial to implement? -- Tim ============================= Tim Bird Architecture Group Chair, CE Workgroup of the Linux Foundation Senior Staff Engineer, Sony Network Entertainment ============================= -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>