On Mon, Mar 25, 2013 at 8:17 PM, <Valdis.Kletnieks@xxxxxx> wrote: > On Mon, 25 Mar 2013 17:23:40 -0700, Raymond Jennings said: > >> Is there some sort of mechanism that throttles the size of the writeback pool? > > There's a lot of tunables in /proc/sys/vm - everything from drop_caches > to swappiness to vfs_cache_pressure. Note that they all interact in mystical > and hard-to-understand ways. ;) I'm pretty familiar with this directory, but alas, I can find nothing regarding writeback throttling that would limit the amount of data in the "writeback" pool. So again I ask, where is it? Unless you are hinting I should search the source myself ^^. >> it's somewhat related to my brainfuck queue, since I would like to >> stress test it digesting a huge pile of outbound data and seeing if it >> can make writeback less seeky. > > The biggest challenge here is that there's a bit of a layering violation > to be resolved - when the VM is choosing what pages get written out first, > it really has no clue where on disk the pages are going. Already realized this myself ^^ > Consider a 16M > file that's fragged into 16 1M extents - they'll almost certainly hit > the writeback queue in logical block order, not physical address order. > The only really good choices here are to either allow the writeback queue > to get deep enough that an elevator can do something useful (if you only > have 2-3 IOs queued, you can do less than if you have 20-30 of them you > can sort into some useful order), and filesystems that are less prone > to fragmentation issues Indeed, the filesystem really ought to be the one making decisions on what to flush, and should be taking hints from the block layer given a sector number. > Just for the record, most of my high-performance stuff runs best with > the noop scheduler - when you're striping I/O across several hundred disks, > the last thing you want is some some single-minded disk scheduler re-arranging > the I/Os and creating latency issues for your striping. > Might want to think about why there's lots of man-hours spent doing > new filesystems and stuff like zcache and kernel shared memory, but the > only IO schedulers in tree are noop, deadline, and cfq :) Hey, gotta cut my teeth somewhere. :) _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
