Messages in this thread | | | Subject | Re: RFC - how to balance Dirty+Writeback in the face of slow writeback. | From | Trond Myklebust <> | Date | Thu, 17 Aug 2006 09:21:51 -0400 |
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On Thu, 2006-08-17 at 13:59 +1000, Neil Brown wrote: > 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.
Exactly how would a request limit help? All that boils down to is having the VM monitor global_page_state(NR_FILE_DIRTY) versus monitoring global_page_state(NR_FILE_DIRTY)+global_page_state(NR_WRITEBACK).
Cheers, Trond
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