On 02/11/2016 03:24 AM, Tim Chen wrote: > On Wed, 2016-02-10 at 13:28 -0800, Andrew Morton wrote: > >> >> If a process is unmapping 4MB then it's pretty crazy for us to be >> hitting the percpu_counter 32 separate times for that single operation. >> >> Is there some way in which we can batch up the modifications within the >> caller and update the counter less frequently? Perhaps even in a >> single hit? > > I think the problem is the batch size is too small and we overflow > the local counter into the global counter for 4M allocations. > The reason for the small batch size was because we use > percpu_counter_read_positive in __vm_enough_memory and it is not precise > and the error could grow with large batch size. > > Let's switch to the precise __percpu_counter_compare that is > unaffected by batch size. It will do precise comparison and only add up > the local per cpu counters when the global count is not precise > enough. > I'm not certain about this. for_each_online_cpu() under spinlock somewhat doubtful. And if we are close to limit we will be hitting slowpath all the time. > So maybe something like the following patch with a relaxed batch size. > I have not tested this patch much other than compiling and booting > the kernel. I wonder if this works for Andrey. We could relax the batch > size further, but that will mean that we will incur the overhead > of summing the per cpu counters earlier when the global count get close > to the allowed limit. > > Thanks. > > Tim > -- 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>