3 ms·

It looks like the mmap call is not within the timed region in do_mmap_test. I'd call that a major flaw in the test methodology when combined with MAP_POPULATE a

It looks like the mmap call is not within the timed region in do_mmap_test. I'd call that a major flaw in the test methodology when combined with MAP_POPULATE and a cold cache. The read syscall version has to actually do IO in the timed region; the mmap(MAP_POPULATE) version just does TLB misses and memcpy. edit: actually, sorry, I think the article points this out, mostly correctly: > We will come back to __memmove_avx_unaligned_erms in a moment, but for the time being let’s try to figure out exactly how much time is spent mapping pages. I have a neat trick up my sleeve to do that. On Linux, the mmap system call can accept a MAP_POPULATE flag. What this flag does is it forces mmap to pre-populate all the page mappings during the actual system call, so none of the page-mapping work would be done when my test actually runs. So I changed my test to invoke mmap with MAP_POPULATE and learned that the experiment completes about 36% faster. (I only measure the timing of the main loop, and not that of the mmap system call). Therefore, I assume that in the above profile all those mapping functions take up about 36%. They didn't point out that it doesn't actually do any IO in the main loop either then, but maybe they were only running the warm cache variant by here anyway. And it seems like they didn't use MAP_POPULATE when generating the comparison graphs earlier in the article.

jschwartzi 6y ago
Methodologically it makes sense to exclude work done in the mmap call. After all you would do that once to initialize the memory but you never need to call it to do the work. The point of the test wasn't to check performance of the mmap syscall versus the read syscall, but simply to check the performance of read() versus memory-mapped files.
- scottlamb 6y ago
  To check the performance of read() versus memory-mapped files with a cold cache, you should absolutely be including the actual IO. But on my closer read, they did that earlier in the article. Here they are examining the (userspace) memcpy in isolation. MAP_POPULATE is just a tool to help with that. They could get a similar effect by just looking at the second (on to Nth) call to do_mmap_test, not the first. Or just copying from heap memory to heap memory, without involving any kernel stuff at all...

jschwartzi 6y ago

Methodologically it makes sense to exclude work done in the mmap call. After all you would do that once to initialize the memory but you never need to call it to do the work. The point of the test wasn't to check performance of the mmap syscall versus the read syscall, but simply to check the performance of read() versus memory-mapped files.

scottlamb 6y ago
To check the performance of read() versus memory-mapped files with a cold cache, you should absolutely be including the actual IO. But on my closer read, they did that earlier in the article. Here they are examining the (userspace) memcpy in isolation. MAP_POPULATE is just a tool to help with that. They could get a similar effect by just looking at the second (on to Nth) call to do_mmap_test, not the first. Or just copying from heap memory to heap memory, without involving any kernel stuff at all...