Hi,
I was using fio and noticed that the throughput was lower than expected for
random reads to a small file; I would expect it to be entirely cached in
memory. The issue seems to be because fadvise_hint=0 is not behaving as
documented as POSIX_FADV_DONTNEED is being called.
This might be the intended bahavior as the program is flushing the pages
to purely test IO throughput
If the current behaviour is intentional, is there an existing flag that
could
provide the desired functionality? Essentially I'd like to test the IO
throughput of various file sizes without bypassing the page cache to see the
effects of caching.
To reproduce:
Version:
Built from source from master (commit f3057d268d)
Output of /fio --version
fio-3.28-18-gf3057
We can see fadvise is being used when ran with strace:
isawan@hypermachine:~/Software/fio$ strace -f -e fadvise64 ./fio
--name=test-reader --ioengine=mmap --readwrite=randread --readonly
--filename=/tmp/test --bs=4k --size=128M --numjobs=1 --runtime=30s
--time_based --buffered=1 --allow_file_create=0 --fadvise_hint=0
test-reader: (g=0): rw=randread, bs=(R) 4096B-4096B, (W)
4096B-4096B, (T) 4096B-4096B, ioengine=mmap, iodepth=1
fio-3.28-18-gf3057
strace: Process 28297 attached
strace: Process 28298 attached
strace: Process 28299 attached
strace: Process 28300 attached
strace: Process 28301 attached
strace: Process 28302 attached
strace: Process 28303 attached
strace: Process 28304 attached
[pid 28301] +++ exited with 0 +++
[pid 28297] +++ exited with 0 +++
[pid 28303] +++ exited with 0 +++
[pid 28298] +++ exited with 0 +++
[pid 28299] +++ exited with 0 +++
[pid 28304] +++ exited with 0 +++
[pid 28302] +++ exited with 0 +++
[pid 28300] +++ exited with 0 +++
strace: Process 28305 attached
Starting 1 process
strace: Process 28306 attached
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
[pid 28306] fadvise64(6, 0, 134217728, POSIX_FADV_DONTNEED) = 0
As we see here, fadvise is being called, while the documentation states:
fadvise_hint=str
Use posix_fadvise(2) or posix_fadvise(2) to advise the kernel
on what I/O patterns are likely to be issued.
Accepted values are:
"0": Backwards-compatible hint for “no hint”
