On Wed 30-11-16 10:16:53, Marc MERLIN wrote: > +folks from linux-mm thread for your suggestion > > On Wed, Nov 30, 2016 at 01:00:45PM -0500, Austin S. Hemmelgarn wrote: > > > swraid5 < bcache < dmcrypt < btrfs > > > > > > Copying with btrfs send/receive causes massive hangs on the system. > > > Please see this explanation from Linus on why the workaround was > > > suggested: > > > https://lkml.org/lkml/2016/11/29/667 > > And Linux' assessment is absolutely correct (at least, the general > > assessment is, I have no idea about btrfs_start_shared_extent, but I'm more > > than willing to bet he's correct that that's the culprit). > > > > All of this mostly went away with Linus' suggestion: > > > echo 2 > /proc/sys/vm/dirty_ratio > > > echo 1 > /proc/sys/vm/dirty_background_ratio > > > > > > But that's hiding the symptom which I think is that btrfs is piling up too many I/O > > > requests during btrfs send/receive and btrfs scrub (probably balance too) and not > > > looking at resulting impact to system health. > > > I see pretty much identical behavior using any number of other storage > > configurations on a USB 2.0 flash drive connected to a system with 16GB of > > RAM with the default dirty ratios because it's trying to cache up to 3.2GB > > of data for writeback. While BTRFS is doing highly sub-optimal things here, > > the ancient default writeback ratios are just as much a culprit. I would > > suggest that get changed to 200MB or 20% of RAM, whichever is smaller, which > > would give overall almost identical behavior to x86-32, which in turn works > > reasonably well for most cases. I sadly don't have the time, patience, or > > expertise to write up such a patch myself though. > > Dear linux-mm folks, is that something you could consider (changing the > dirty_ratio defaults) given that it affects at least bcache and btrfs > (with or without bcache)? As much as the dirty_*ratio defaults a major PITA this is not something that would be _easy_ to change without high risks of regressions. This topic has been discussed many times with many good ideas, nothing really materialized from them though :/ To be honest I really do hate dirty_*ratio and have seen many issues on very large machines and always suggested to use dirty_bytes instead but a particular value has always been a challenge to get right. It has always been very workload specific. That being said this is something more for IO people than MM IMHO. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>