On Thu, Jun 30, 2011 at 04:04:59PM -0400, Vivek Goyal wrote: [..] > Dave, > > Just another example where serialization is taking place with ext4. > > I created a group with 1MB/s write limit and ran tedso's fsync tester > program with little modification. I used write() system call instead > of pwrite() so that file size grows. This program basically writes > 1MB of data and then fsync's it and then measures the fsync time. > > I ran two instances of prgram in two groups on two separate files. One > instances is throttled to 1MB/s and other is in root group unthrottled. > > Unthrottled program gets serialized behind throttled one. Following > are fsync times. > > Throttled instance Unthrottled Instance > ------------------ -------------------- > fsync time: 1.0051 fsync time: 1.0067 > fsync time: 1.0049 fsync time: 1.0075 > fsync time: 1.0048 fsync time: 1.0063 > fsync time: 1.0073 fsync time: 1.0062 > fsync time: 1.0070 fsync time: 1.0078 > fsync time: 1.0032 fsync time: 1.0049 > fsync time: 0.0154 fsync time: 1.0068 > fsync time: 0.0137 fsync time: 1.0048 > > Without any throttling both the instances do fine > ------------------------------------------------- > Throttled instance Unthrottled Instance > ------------------ -------------------- > fsync time: 0.0139 fsync time: 0.0162 > fsync time: 0.0132 fsync time: 0.0156 > fsync time: 0.0149 fsync time: 0.0169 > fsync time: 0.0165 fsync time: 0.0152 > fsync time: 0.0188 fsync time: 0.0135 > fsync time: 0.0137 fsync time: 0.0142 > fsync time: 0.0148 fsync time: 0.0149 > fsync time: 0.0168 fsync time: 0.0163 > fsync time: 0.0153 fsync time: 0.0143 > > So when we are inreasing the size of file and fsyncing it, other > unthrottled instances of similar activities will get throttled > behind it. > > IMHO, this is a problem and should be fixed. If filesystem can fix it great. > But if not, then we should consider the option of throttling buffered writes > in balance_dirty_pages(). XFS seems to be doing well for this particular test. Unthrottled fsyncer does not get serialized behind throttled one. Throttled instance Unthrottled Instance ------------------ -------------------- fsync time: 1.0511 fsync time: 0.0204 fsync time: 1.0486 fsync time: 0.0260 fsync time: 1.0445 fsync time: 0.0260 fsync time: 1.0485 fsync time: 0.0260 fsync time: 1.0446 fsync time: 0.0260 fsync time: 1.2157 fsync time: 0.0260 fsync time: 1.0446 fsync time: 0.0300 fsync time: 1.0484 fsync time: 0.0340 fsync time: 1.0446 fsync time: 0.0221 fsync time: 1.0486 fsync time: 0.0340 fsync time: 1.0406 fsync time: 0.0340 Thanks Vivek -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
