OK for reply to all. I'm using the first mail received. > Interesting that data=writeback helped this. You don't give a lot of > details, but I assume that data=writeback made a large difference here? Well, my measures were taken by counting seconds from the desktop clock, so the precision is roughly 1s. Here are the effective results from the 3 journaling modes I have done yesterday : Times in seconds: ordered mode : 17 5 38 19 33 journal mode : 6 1 5 6 5 writeback mode : 4 1 7 6 8 I do not remember exactly the operation sequence, but the beginning was opening Konqueror, opening the bookmarks menu, loading a page, and opening another tab. As you can see anyway, ordered is completely broken, opening the menu is near 1s in journal and writeback mode, which is perfectly acceptable for a user (after all, the computer is loaded, and interactivity is not simply running interactive tasks at real time priority, so the the user can understand it reacts slower). Launching Konqueror in 4 to 6 seconds is a good time, given that the cache is poisoned by the writing process. Opening another tab or a web page is tolerable within 5-6s, even if this is still a little too long, specially for opening another tab of an already running process (2-3s would be a perfect value I think, but this is another story to get this). Certainly 5-6s of latency is due to the block size used by the writing process, and tuning the kernel to get lower results would be dangerous for general performances. I could do tests by asking dd to write more but smaller blocks to check this idea. As far as writeback is concerned, the latency reduction was not clear as you can see, at least with my mesurement procedure. For performance of the **writing process**, it was nearly the same as ordered. I havn't written the timings results for the writing process, since I was still investigating at that time. But I used : time dowrite.sh to time it, and I got real time, as well as dd output (records written, MB/s, and journal was half as ordered). So writeback and ordered seemed the same at this level, and journal mode was effectively twice slower. It was clear there was twice as most write operations, that was the expected behaviour. I could redo the tests, with a more formal approach (that is: fixing the operations order done in Konqueror, and noting the times for the writing process), it's easy. In fact, I'm considering doing another script simulating Konqueror / user activities and timing them automatically, just to get a reproduceable benchmark suite. Please note that not all programs are affected (for instance kmail was quite quick), so the kernel does a good job to run programs that do not many I/Os when the fs is stressed by another process. The difference with konqueror is it is opening several files (block dump says), so my guess is that processes doing small I/O operations are not given "journal access" fairly enough in ordered mode. Indeed, I imagine (without knowing any internals of course) that the data writing is indeed scheduled correctly by the disk scheduler, and that the writing process is throttled, but the priority informations are lost when metadata are written in ordered mode, because all I/Os are mixed for metadata. Therefore, the writing process manages to fill the journal quite easily. With journal and writeback mode, all data may be written sequentially, in the context of the issuing process, so priorities may be tracked on the whole writing process. Then the writing process is prempted correctly to the benefits of Konqueror. This is of course just "black box" hypothesis... CC - 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
