On 14.11.18 г. 9:17 ч., Qu Wenruo wrote: > > > On 2018/11/14 下午3:00, Qu Wenruo wrote: >> >> >> On 2018/11/14 下午2:47, Nikolay Borisov wrote: >>> >>> >>> On 14.11.18 г. 2:31 ч., Qu Wenruo wrote: >>>> Hi, >>>> >>>> Is there any (easy) method for a fstests test case to limit the page >>>> cache usage? >>>> >>>> I triggered btrfs/139 failure with 2G vRAM VM, and located the root >>>> cause of the problem. >>> >>> You can always size your test vm properly. Otherwise what about the >>> various sysctl tuning knobs? I.e Documentation/sysctl/vm.txt explains >>> some of them: dirty_bytes, dirty_background_bytes, >>> dirty_background_ratio, dirty_expire_centisecs >> >> Thanks for the hint about vm.txt! >> >> I just realized we could just drop_caches to force dirty page writeback, >> without the need to tweaking the complex memory pressure/watermark >> mechanism. > > Well, this doesn't work as expected. > > It will cause transaction commit, seems that kernel is trying too hard > to free page cache. > > Is there any way to only flush dirty pages of a file? well fdatasync causes ->fsync to run, in btrfs that will be btrfs_sync_file. One of the first thing it does is start_ordered_ops which does btrfs_fdatawrite_range, which in turn is filemap_fdatawrite_range > > Thanks, > Qu > >> >> Thanks, >> Qu >>> >>> >>> So with a 2g machine the default settings are using only a fraction of >>> the ram. If you adjust the same settings for the larger ram size you >>> should get almost identical behavior.> >>> >>> >>>> >>>> However it's only really reproducible for small ram VM, since it could >>>> trigger dirty page writeback due to memory pressure. >>>> >>>> So I'm wondering if we could do such thing even for large RAM test machine. >>>> >>>> Thanks, >>>> Qu >>>> >
