zram: support asynchronous GC for lazy slot freeing#735
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Swap freeing can be expensive when unmapping a VMA containing many swap entries. This has been reported to significantly delay memory reclamation during Android’s low-memory killing, especially when multiple processes are terminated to free memory, with slot_free() accounting for more than 80% of the total cost of freeing swap entries. Two earlier attempts by Lei and Zhiguo added a new thread in the mm core to asynchronously collect and free swap entries [1][2], but the design itself is fairly complex. When anon folios and swap entries are mixed within a process, reclaiming anon folios from killed processes helps return memory to the system as quickly as possible, so that newly launched applications can satisfy their memory demands. It is not ideal for swap freeing to block anon folio freeing. On the other hand, swap freeing can still return memory to the system, although at a slower rate due to memory compression. Therefore, in zram, we introduce a GC worker to allow anon folio freeing and slot_free to run in parallel, since slot_free is performed asynchronously, maximizing the rate at which memory is returned to the system. Xueyuan’s test on RK3588 shows that unmapping a 256MB swap-filled VMA becomes 3.4× faster when pinning tasks to CPU2, reducing the execution time from 63,102,982 ns to 18,570,726 ns. A positive side effect is that async GC also slightly improves do_swap_page() performance, as it no longer has to wait for slot_free() to complete. Xueyuan’s test shows that swapping in 256MB of data (each page filled with repeating patterns such as “1024 one”, “1024 two”, “1024 three”, and “1024 four”) reduces execution time from 1,358,133,886 ns to 1,104,315,986 ns, achieving a 1.22× speedup. [1] https://lore.kernel.org/all/20240805153639.1057-1-justinjiang@vivo.com/ [2] https://lore.kernel.org/all/20250909065349.574894-1-liulei.rjpt@vivo.com/ Tested-by: Xueyuan Chen <xueyuan.chen21@gmail.com> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
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Pull request for series with
subject: zram: support asynchronous GC for lazy slot freeing
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1080269