On Thu 18-10-12 16:13:58, Chris Friesen wrote: > On 10/18/2012 03:28 PM, Jan Kara wrote: > > > Yeah, ionice has its limitations. The problem is that all buffered > >writes happen just into memory (so completely independently of ionice > >settings). Subsequent writing of dirty memory to disk happens using flusher > >thread which is a kernel process and it doesn't know anything about IO > >priority set for task which created the file. If you wrote the file with > >oflag=direct or oflag=sync you would see that ionice works as expected. > > Has anyone looked at storing the ionice value with the buffered > write request such that the actual writes to disk could be sorted by > priority and done with the ionice level of the original caller? There's nothing as "buffered write request" in kernel. When buffered write happens, data are just copied into page cache. We could attach a tag to each modified page in the page cache but that would get really expensive. Essentially the same problems happens with cgroups where buffered writes are not accounted as well. There we considered to attach a tag to inodes (which doesn't work well if processes from different cgroups / with different IO priority write to the same inode but that's not that common) which is reasonably cheap. But then you have to build smarts into flusher thread to prioritize inodes according to tags (you cannot really let flusher thread just submit IO with that priority because when it gets blocked, it starves writeback with possible higher priority). Alternatively you could have separate flusher thread per-cgroup / IO priority. That is easier from code point of view but throughput suffers because of limited merging of IO. So all in all the problem is known but hard to tackle. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
