John Hubbard <jhubbard@xxxxxxxxxx> writes: > On 03/23/2017 09:52 PM, Huang, Ying wrote: >> John Hubbard <jhubbard@xxxxxxxxxx> writes: >> >>> On 03/23/2017 07:41 PM, Huang, Ying wrote: >>>> David Rientjes <rientjes@xxxxxxxxxx> writes: >>>> >>>>> On Mon, 20 Mar 2017, Huang, Ying wrote: >>>>> >>>>>> From: Huang Ying <ying.huang@xxxxxxxxx> >>>>>> >>>>>> Now vzalloc() is used in swap code to allocate various data >>>>>> structures, such as swap cache, swap slots cache, cluster info, etc. >>>>>> Because the size may be too large on some system, so that normal >>>>>> kzalloc() may fail. But using kzalloc() has some advantages, for >>>>>> example, less memory fragmentation, less TLB pressure, etc. So change >>>>>> the data structure allocation in swap code to use kvzalloc() which >>>>>> will try kzalloc() firstly, and fallback to vzalloc() if kzalloc() >>>>>> failed. >>>>>> >>>>> >>>>> As questioned in -v1 of this patch, what is the benefit of directly >>>>> compacting and reclaiming memory for high-order pages by first preferring >>>>> kmalloc() if this does not require contiguous memory? >>>> >>>> The memory allocation here is only for swap on time, not for swap out/in >>>> time. The performance of swap on is not considered critical. But if >>>> the kmalloc() is used instead of the vmalloc(), the swap out/in >>>> performance could be improved (marginally). More importantly, the >>>> interference for the other activity on the system could be reduced, For >>>> example, less memory fragmentation, less TLB usage of swap subsystem, >>>> etc. >>> >>> Hi Ying, >>> >>> I'm a little surprised to see vmalloc calls replaced with >>> kmalloc-then-vmalloc calls, because that actually makes fragmentation >>> worse (contrary to the above claim). That's because you will consume >>> contiguous memory (even though you don't need it to be contiguous), >>> whereas before, you would have been able to get by with page-at-a-time >>> for vmalloc. >>> >>> So, things like THP will find fewer contiguous chunks, as a result of patches such as this. >> >> Hi, John, >> >> I don't think so. The pages allocated by vmalloc() cannot be moved >> during de-fragment. For example, if 512 dis-continuous physical pages >> are allocated via vmalloc(), at worst, one page will be allocate from >> one distinct 2MB continous physical pages. This makes 512 * 2MB = 1GB >> memory cannot be used for THP allocation. Because these pages cannot be >> defragmented until vfree(). > > kmalloc requires a resource that vmalloc does not: contiguous > pages. Therefore, given the same mix of pages (some groups of > contiguous pages, and a scattering of isolated single-page, or > too-small-to-satisfy-entire-alloc groups of pages, and the same > underlying page allocator, kmalloc *must* consume the more valuable > contiguous pages. However, vmalloc *may* consume those same pages. > > So, if you run kmalloc a bunch of times, with higher-order requests, > you *will* run out of contiguous pages (until more are freed up). If > you run vmalloc with the same initial conditions and the same > requests, you may not necessary use up those contiguous pages. > > It's true that there are benefits to doing a kmalloc-then-vmalloc, of > course: if the pages are available, it's faster and uses less > resources. Yes. I just don't think "less fragmentation" should be > listed as a benefit, because you can definitely cause *more* > fragmentation if you use up contiguous blocks unnecessarily. Yes, I agree that for some cases, kmalloc() will use more contiguous blocks, for example, non-movable pages are scattered all over the memory. But I still think in common cases, if defragement is enabled, and non-movable pages allocation is restricted to some memory area if possible, kmalloc() is better than vmalloc() as for fragmentation. Best Regards, Huang, Ying > -- > thanks, > john h -- 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>
