On Wed, 11 Jan 2017 09:55:13 -0800 Tim Chen <tim.c.c...@linux.intel.com> wrote:
> The patch is to improve the scalability of the swap out/in via using > fine grained locks for the swap cache. In current kernel, one address > space will be used for each swap device. And in the common > configuration, the number of the swap device is very small (one is > typical). This causes the heavy lock contention on the radix tree of > the address space if multiple tasks swap out/in concurrently. But in > fact, there is no dependency between pages in the swap cache. So that, > we can split the one shared address space for each swap device into > several address spaces to reduce the lock contention. In the patch, the > shared address space is split into 64MB trunks. 64MB is chosen to > balance the memory space usage and effect of lock contention reduction. > > The size of struct address_space on x86_64 architecture is 408B, so with > the patch, 6528B more memory will be used for every 1GB swap space on > x86_64 architecture. > > One address space is still shared for the swap entries in the same 64M > trunks. To avoid lock contention for the first round of swap space > allocation, the order of the swap clusters in the initial free clusters > list is changed. The swap space distance between the consecutive swap > clusters in the free cluster list is at least 64M. After the first > round of allocation, the swap clusters are expected to be freed > randomly, so the lock contention should be reduced effectively. Switching from a single radix-tree to an array of radix-trees to reduce contention seems a bit hacky. That we can do this and have everything continue to work tells me that we're simply using an inappropriate data structure to hold this info.
