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On Tuesday May 30, nickpiggin@yahoo.com.au wrote: > Nick Piggin wrote: > > Linus Torvalds wrote: > > > >> > >> Why do you think the IO layer should get larger requests? > > > > > > For workloads where plugging helps (ie. lots of smaller, contiguous > > requests going into the IO layer), should be pretty good these days > > due to multiple readahead and writeback. > > Let me try again. > > For workloads where plugging helps (ie. lots of smaller, contiguous > requests going into the IO layer), the request pattern should be > pretty good without plugging these days, due to multiple page > readahead and writeback. Can I please put in a vote for not thinking that every device is disk drive? I find plugging fairly important for raid5, particularly for write. The more whole-stripe writes I can get, the better throughput I get. So I tend to keep a raid5 array plugged while any requests are arriving, and interpret 'plugged' to mean that incomplete stripes don't get processed while full stripes (needing no pre-reading) do get processed. The only way "large requests" are going to replace plugging is they are perfectly aligned, which I don't expect to ever see. As for your original problem.... I wonder if PG_locked is protecting too much? It protects against IO and it also protects against ->mapping changes. So if you want to ensure that ->mapping won't change, you need to wait for any pending read request to finish, which seems a bit dumb. Maybe we need a new bit: PG_maplocked. You are only allowed to change ->mapping or ->index of you hold PG_locked and PG_maplocked, you are not allowed to wait for PG_locked while holding PG_maplocked, and you can read ->mapping or ->index while PG_locked or PG_maplocked are held. Think of PG_locked like a mutex and PG_maplocked like a spinlock (and probably use bit_spinlock to get it). Then set_page_dirty_lock would use PG_maplocked to get access to ->mapping, and then hold a reference on the address_space while calling into balance_dirty_pages ... I wonder how you hold a reference on an address space... There are presumably few pieces of code that change ->mapping. Once they all take PG_maplocked as well as PG_locked, you can start freeing up other code to take PG_maplocked instead of PG_locked.... Does that make sense at all? Do we have any spare page bits? NeilBrown - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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