On Fri, 13 Jun 2014, Lukas Czerner wrote: > Date: Fri, 13 Jun 2014 15:55:35 +0200 > From: Lukas Czerner <lczerner@xxxxxxxxxx> > To: linux-ext4@xxxxxxxxxxxxxxx > Subject: [RFC][PATCH 0/1] ext4: Fix ext4_mb_normalize_request > > This is my first attempt to fix the ext4_mb_normalize_request() function in > in ext4 which deals with file preallocations. > > This is not yet a final version as it needs more testing, however I'd like > to see some suggestions. Does anyone have any comments on this and the related patch ? Thanks! -Lukas > > > Currently there are couple of problems with ext4_mb_normalize_request(). > > - We're trying to normalize unwritten extents allocation which is > entirely unnecessary, because user exactly knows what how much space > he is going to need - no need for file system to do preallocations. > > - ext4_mb_normalize_request() unnecessarily divides bigger allocation > requests to small ones (8MB). I believe that this is a bug rather than > design. > > - For smaller allocations (or smaller files) we do not even respect the > fe_logical. Although we do respect it for bigger files. > > - Overall the logic within ext4_mb_normalize_request() is weird and > no-one really understand why it is the way it is. > > Fix all of this by: > > - Disabling preallocation for unwritten extent allocation. However > because the maximum size of the unwritten extent is one block smaller > than written, in order to avoid unnecessary fragmentation we limit the > request to EXT_INIT_MAX_LEN / 2 > > - Get rid of the "if table" in ext4_mb_normalize_request() and replace > it with simply aligning the assumed end of the file up to power of > two. But we still limit the allocation size to EXT4_BLOCKS_PER_GROUP. > Also do this on file system block units to take into account different > block sized file systems. > > > It passes xfstests cleanly in default configuration, I've not tried any > non-default options yet. > > I've tried to test how much it changes allocation. The test and some results > can be found at > > http://people.redhat.com/lczerner/mballoc/ > > normalize.sh is the simple script I run and output.normalize_orig[34] > contains result from the vanila 3.15.0 while output.normalize_patch[56] > contains results with this patch. > > From the performance stand point I do not see any major differences except > that untar seems to always generate better results (which might be because > of bigger continuous extents). > > Free space fragmentation seems to be about the same, however with the patch > there seems to be less smaller free space extents and more bigger ones which > is expected due to bigger preallocations (and I think it's a good thing). > > The biggest difference which is obvious from the results is that extent tree > is much smaller (sometimes five times smaller) with the patch. Except of the > fallocate case because we now limit the requests to (EXT_INIT_MAX_LEN / 2) > so we can not merge them - it might be worth experimenting with something > smaller which is a factor of unwritten extent size. > > But as I said the extent tree is much smaller which means that the extents > overall are bigger which again is a good thing. This becomes very obvious > when we look at the extent tree of the image file (the last steps in the > test). > > What do you think ? > > Thanks! > -Lukas > > > > -- > 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 > -- 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
