On 2011-05-20 16:42, Animesh K Trivedi1 wrote: > > Hi all, > > I'm running the random read test on a 64MB file with 4K block size. The > file is small enough to fit in the buffer cache of the system (8 GB DRAM). > But default fadvise calls to OS pass two flags (POSIX_FADV_DONTNEED, and > POSIX_FADV_RANDOM). With these two settings OS does not try to cache any > reads and they all go to the disk. I get ~280 IOPS with 99% disk > utilization. And that is fine. But even when I turn fadvise_hint=0, it > shows the similar behaviour. Upon further investigation I noticed that > irrespective of settings, POSIX_FADV_DONTNEED is always passed to OS > (filesetup.c, line 361). Should not it also be restricted when > --fadvise_hint is set to zero ? FADV_DONTNEED is used to invalidate the mapping cache so that buffered runs are "identical" and not influenced by previous cache hotness of the page cache. FADV_DONTNEED should not have any effects on the caching later done, whereas FADV_RANDOM definitely will impact caching or read-ahead. > If I don't pass any hint to OS, then after stabilizing whole file is > cached in the buffer and IOPS reaches upto 700+K (which I was expecting for > small file sizes). Here is the snipped sample output : > > fio --fadvise_hint=0 --ioengine=psync --direct=0 --rw=randread --bs=4K > --size=64M --numjob=1 --runtime=60 --time_based --group_reporting > --name=file > file: (g=0): rw=randread, bs=4K-4K/4K-4K, ioengine=psync, iodepth=1 > fio 1.54 > Starting 1 process > Jobs: 1 (f=1): [r] [100.0% done] [3043M/0K /s] [761K/0 iops] [eta 00m:00s] > file: (groupid=0, jobs=1): err= 0: pid=3820 > read : io=121156MB, bw=2019.3MB/s, iops=516931 , runt= 60000msec > ...... > Disk stats (read/write): > sda: ios=7742/82, merge=0/4060, ticks=34608/9112, in_queue=43712, > util=32.27% > > Am I missing something ? You can always disable the cache clear hint with invalidate=0. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
