On Wed, Apr 14, 2010 at 09:51:33AM +0100, Mel Gorman wrote: > They will need to be tackled in turn then but obviously there should be > a focus on the common paths. The reclaim paths do seem particularly > heavy and it's down to a lot of temporary variables. I might not get the > time today but what I'm going to try do some time this week is > > o Look at what temporary variables are copies of other pieces of information > o See what variables live for the duration of reclaim but are not needed > for all of it (i.e. uninline parts of it so variables do not persist) > o See if it's possible to dynamically allocate scan_control > > The last one is the trickiest. Basically, the idea would be to move as much > into scan_control as possible. Then, instead of allocating it on the stack, > allocate a fixed number of them at boot-time (NR_CPU probably) protected by > a semaphore. Limit the number of direct reclaimers that can be active at a > time to the number of scan_control variables. kswapd could still allocate > its on the stack or with kmalloc. > > If it works out, it would have two main benefits. Limits the number of > processes in direct reclaim - if there is NR_CPU-worth of proceses in direct > reclaim, there is too much going on. It would also shrink the stack usage > particularly if some of the stack variables are moved into scan_control. > > Maybe someone will beat me to looking at the feasibility of this. I already have some patches to remove trivial parts of struct scan_control, namely may_unmap, may_swap, all_unreclaimable and isolate_pages. The rest needs a deeper look. A rather big offender in there is the combination of shrink_active_list (360 bytes here) and shrink_page_list (200 bytes). I am currently looking at breaking out all the accounting stuff from shrink_active_list into a separate leaf function so that the stack footprint does not add up. Your idea of per-cpu allocated scan controls reminds me of an idea I have had for some time now: moving reclaim into its own threads (per cpu?). Not only would it separate the allocator's stack from the writeback stack, we could also get rid of that too_many_isolated() workaround and coordinate reclaim work better to prevent overreclaim. But that is not a quick fix either... -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
