Hi Huang, Ccing Johannes: I don't read this patch yet but I remember Johannes tried VMA-based readahead approach long time ago so he might have good comment. On Fri, Jun 30, 2017 at 09:44:37AM +0800, Huang, Ying wrote: > The swap readahead is an important mechanism to reduce the swap in > latency. Although pure sequential memory access pattern isn't very > popular for anonymous memory, the space locality is still considered > valid. > > In the original swap readahead implementation, the consecutive blocks > in swap device are readahead based on the global space locality > estimation. But the consecutive blocks in swap device just reflect > the order of page reclaiming, don't necessarily reflect the access > pattern in virtual memory space. And the different tasks in the > system may have different access patterns, which makes the global > space locality estimation incorrect. > > In this patchset, when page fault occurs, the virtual pages near the > fault address will be readahead instead of the swap slots near the > fault swap slot in swap device. This avoid to readahead the unrelated > swap slots. At the same time, the swap readahead is changed to work > on per-VMA from globally. So that the different access patterns of > the different VMAs could be distinguished, and the different readahead > policy could be applied accordingly. The original core readahead > detection and scaling algorithm is reused, because it is an effect > algorithm to detect the space locality. > > In addition to the swap readahead changes, some new sysfs interface is > added to show the efficiency of the readahead algorithm and some other > swap statistics. > > This new implementation will incur more small random read, on SSD, the > improved correctness of estimation and readahead target should beat > the potential increased overhead, this is also illustrated in the test > results below. But on HDD, the overhead may beat the benefit, so the > original implementation will be used by default. > > The test and result is as follow, > > Common test condition > ===================== > > Test Machine: Xeon E5 v3 (2 sockets, 72 threads, 32G RAM) > Swap device: NVMe disk > > Micro-benchmark with combined access pattern > ============================================ > > vm-scalability, sequential swap test case, 4 processes to eat 50G > virtual memory space, repeat the sequential memory writing until 300 > seconds. The first round writing will trigger swap out, the following > rounds will trigger sequential swap in and out. > > At the same time, run vm-scalability random swap test case in > background, 8 processes to eat 30G virtual memory space, repeat the > random memory write until 300 seconds. This will trigger random > swap-in in the background. > > This is a combined workload with sequential and random memory > accessing at the same time. The result (for sequential workload) is > as follow, > > Base Optimized > ---- --------- > throughput 345413 KB/s 414029 KB/s (+19.9%) > latency.average 97.14 us 61.06 us (-37.1%) > latency.50th 2 us 1 us > latency.60th 2 us 1 us > latency.70th 98 us 2 us > latency.80th 160 us 2 us > latency.90th 260 us 217 us > latency.95th 346 us 369 us > latency.99th 1.34 ms 1.09 ms > ra_hit% 52.69% 99.98% > > The original swap readahead algorithm is confused by the background > random access workload, so readahead hit rate is lower. The VMA-base > readahead algorithm works much better. > > Linpack > ======= > > The test memory size is bigger than RAM to trigger swapping. > > Base Optimized > ---- --------- > elapsed_time 393.49 s 329.88 s (-16.2%) > ra_hit% 86.21% 98.82% > > The score of base and optimized kernel hasn't visible changes. But > the elapsed time reduced and readahead hit rate improved, so the > optimized kernel runs better for startup and tear down stages. And > the absolute value of readahead hit rate is high, shows that the space > locality is still valid in some practical workloads. -- 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>
