Messages in this thread | | | Date | Sat, 23 Feb 2002 13:06:36 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] only irq-safe atomic ops |
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Andi Kleen wrote: > > Andrew Morton <akpm@zip.com.au> writes: > > > > > > > I can tell you that Irix has just such a global counter for the amount of > > > delayed allocate pages - and it gets to be a major point of cache contention > > > once you get to larger cpu counts. So avoiding that from the start would > > > be good. > > > > Ah, good info. Thanks. I'll fix it with a big "FIXME" comment for now, > > fix it for real when Rusty's per-CPU infrastructure appears. > > Just curious -- how do you want to fix it for real? > As far as I can see a delalloc counter needs to be exact to avoid OOM > deadlocks, but making it per CPU would require doing the accounting inexact.
The counter is used only for making writeback decisions. It is completely analogous to nr_buffers_type[BUF_DIRTY], except it counts pages. So it does not need to be exact for readers.
If we needed exact reader-accounting for the number of dirty pages in the machine then we'd need a ton of new locking in fun places like __free_pte(), and that still doesn't account for pages which are only pte-dirty, and it's not obvious what we'd do with reader-exact dirty page info anyway?
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