Re: [Crash-utility] [PATCH 0/5] [RFC] Multi-thread support for search cmd

The primary part of the patchset will introduce multithread support for search cmd to improve its performance. A search operation is mainly made up with 2 steps: 1) readmem data into pagebuf, 2) search specific values within the pagebuf. A typical workflow of search is as follows: for addr from low to high: do readmem(addr, pagebuf) search_value(value, pagebuf) addr += pagesize done There are 2 points which we can accelerate: 1) readmem don't have to wait search_value, when search_value is working, readmem can read the next pagebuf at the same time. 2) higher addr don't have to wait lower addr, they can be processed at the same time if we carefully arrange the output order. For point 1, we introduce zones for pagebuf, e.g. search_value can work on zone 0 while readmem can prepare the data for zone 1. For point 2, we introduce multiple search_value in threads, e.g. readmem will prepare 100 pages as a batch, then we will have 4 threads of search_value, thread 0 handles page 1~25, thread 2 handles page 26~50 page, thread 3 handles page 51~75, thread 4 handles page 76~100. A typical workflow of multithread search implemented in this patchset is as follows, which removed thread synchronization: pagebuf[ZONE][BATCH] zone_index = buf_index = 0 create_thread(4, search_value) for addr from low to high: do if buf_index < BATCH readmem(addr, pagebuf[zone_index][buf_index++]) addr += pagesize else start_thread(pagebuf[zone_index], 0/4 * BATCH, 1/4 * BATCH) start_thread(pagebuf[zone_index], 1/4 * BATCH, 2/4 * BATCH) start_thread(pagebuf[zone_index], 2/4 * BATCH, 3/4 * BATCH) start_thread(pagebuf[zone_index], 3/4 * BATCH, 4/4 * BATCH) zone_index++ buf_index = 0 fi done readmem works in the main process and not multi-threaded, because readmem will not only read data from vmcore, decompress it, but walk through page tables if virtual address given. It is hard to reimplement it into thread safe version, search_value is easier to be made thread-safe. By carefully choose batch size and thread num, we can maximize the concurrency. The last part of the patchset, is replacing lseek/read to pread for kcore and diskdumped vmcore. Here is the performance test result chart. Please note the vmcore and kcore are tested seperately on 2 different machines. crash-orig is the crash compiled with clean upstream code, crash-pread is the code with only pread patch applied(patch 5), crash-multi is the code with only multithread patches applied(patch 1~4). ulong search: $ time echo "search abcd" | ./crash-orig vmcore vmlinux > /dev/null $ time echo "search abcd -f 4 -n 4" | ./crash-multi vmcore vmlinux > /dev/null 45G vmcore 64G kcore real user sys real user sys crash-orig 16m56.595s 15m57.188s 0m56.698s 1m37.982s 0m51.625s 0m46.266s crash-pread 16m46.366s 15m55.790s 0m48.894s 1m9.179s 0m36.646s 0m32.368s crash-multi 16m26.713s 19m8.722s 1m29.263s 1m27.661s 0m57.789s 0m54.604s string search: $ time echo "search -c abcddbca" | ./crash-orig vmcore vmlinux > /dev/null $ time echo "search -c abcddbca -f 4 -n 4" | ./crash-multi vmcore vmlinux > /dev/null 45G vmcore 64G kcore real user sys real user sys crash-orig 33m33.481s 32m38.321s 0m52.771s 8m32.034s 7m50.050s 0m41.478s crash-pread 33m25.623s 32m35.019s 0m47.394s 8m4.347s 7m35.352s 0m28.479s crash-multi 16m31.016s 38m27.456s 1m11.048s 5m11.725s 7m54.224s 0m44.186s Discussion: 1) Either multithread and pread patches can improve the performance a bit, so if both patches applied, the performance can be better. 2) Multi-thread search performs much better in search time consumptive tasks, such as string search.