Messages in this thread | | | From | Neil Brown <> | Date | Thu, 17 Aug 2006 13:59:41 +1000 | Subject | Re: RFC - how to balance Dirty+Writeback in the face of slow writeback. |
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On Tuesday August 15, akpm@osdl.org wrote: > > When Dirty hits 0 (and Writeback is theoretically 80% of RAM) > > balance_dirty_pages will no longer be able to flush the full > > 'write_chunk' (1.5 times number of recent dirtied pages) and so will > > spin in a loop calling blk_congestion_wait(WRITE, HZ/10), so it isn't > > a busy loop, but it won't progress. > > This assumes that the queues are unbounded. They're not - they're limited > to 128 requests, which is 60MB or so.
Ahhh... so the limit on the requests-per-queue is an important part of write-throttling behaviour. I didn't know that, thanks.
fs/nfs doesn't seem to impose a limit. It will just allocate as many as you ask for until you start running out of memory. I've seen 60% of memory (10 out of 16Gig) in writeback for NFS.
Maybe I should look there to address my current issue, though imposing a system-wide writeback limit seems safer.
> > Per queue. The scenario you identify can happen if it's spread across > multiple disks simultaneously. > > CFQ used to have 1024 requests and we did have problems with excessive > numbers of writeback pages. I fixed that in 2.6.early, but that seems to > have got lost as well. >
What would you say constitutes "excessive"? Is there any sense in which some absolute number is excessive (as it takes too long to scan some list) or is it just a percent-of-memory thing?
> > Something like that - it'll be relatively simple.
Unfortunately I think it is also relatively simple to get it badly wrong:-) Make one workload fast, and another slower.
But thanks, you've been very helpful (as usual). I'll ponder it a bit longer and see what turns up.
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