| Demonstration of readahead, the Linux eBPF/bcc version |
| |
| Read-ahead mechanism is used by operation sytems to optimize sequential operations |
| by reading ahead some pages to avoid more expensive filesystem operations. This tool |
| shows the performance of the read-ahead caching on the system under a given load to |
| investigate any caching issues. It shows a count for unused pages in the cache and |
| also prints a histogram showing how long they have remianed there. |
| |
| Usage Scenario |
| ============== |
| |
| Consider that you are developing a React Native application which performs aggressive |
| reads while re-encoding a video in local-storage. Usually such an app would be multi- |
| layered and have transitional library dependencies. The actual read may be performed |
| by some unknown native library which may or may not be using hints to the OS, such as |
| madvise(p, LEN, MADV_SEQUENTIAL). If high IOPS is observed in such an app, running |
| readahead may pin the issue much faster in this case as the developer digs deeper |
| into what may be causing this. |
| |
| An example where such an issue can surface is: https://github.com/boltdb/bolt/issues/691 |
| |
| # readahead -d 30 |
| Tracing... Hit Ctrl-C to end. |
| ^C |
| Read-ahead unused pages: 6765 |
| Histogram of read-ahead used page age (ms): |
| |
| age (ms) : count distribution |
| 0 -> 1 : 4236 |****************************************| |
| 2 -> 3 : 394 |*** | |
| 4 -> 7 : 1670 |*************** | |
| 8 -> 15 : 2132 |******************** | |
| 16 -> 31 : 401 |*** | |
| 32 -> 63 : 1256 |*********** | |
| 64 -> 127 : 2352 |********************** | |
| 128 -> 255 : 357 |*** | |
| 256 -> 511 : 369 |*** | |
| 512 -> 1023 : 366 |*** | |
| 1024 -> 2047 : 181 |* | |
| 2048 -> 4095 : 439 |**** | |
| 4096 -> 8191 : 188 |* | |
| |
| In the example above, we recorded system-wide stats for 30 seconds. We can observe that |
| while most of the pages stayed in the readahead cache for quite less time, after 30 |
| seconds 6765 pages still remained in the cache, yet unaccessed. |
| |
| Note on Kprobes Usage |
| ===================== |
| |
| This tool uses Kprobes on the following kernel functions: |
| |
| __do_page_cache_readahead()/do_page_cache_ra() (After kernel version 5.10 (include), __do_page_cache_readahead was renamed to do_page_cache_ra) |
| __page_cache_alloc() |
| mark_page_accessed() |
| |
| Since the tool uses Kprobes, depending on your linux kernel's compilation, these |
| functions may be inlined and hence not available for Kprobes. To see whether you have |
| the functions available, check vmlinux source and binary to confirm whether inlining is |
| happening or not. You can also check /proc/kallsyms on the host and verify if the target |
| functions are present there before using this tool. |
