From e386771cbbc2a90b46df2ace02214f94cc23cb50 Mon Sep 17 00:00:00 2001 From: Andrew Morton Date: Sat, 21 Dec 2002 01:07:33 -0800 Subject: [PATCH] Give kswapd writeback higher priority than pdflush The `low latency page reclaim' design works by preventing page allocators from blocking on request queues (and by preventing them from blocking against writeback of individual pages, but that is immaterial here). This has a problem under some situations. pdflush (or a write(2) caller) could be saturating the queue with highmem pages. This prevents anyone from writing back ZONE_NORMAL pages. We end up doing enormous amounts of scenning. A test case is to mmap(MAP_SHARED) almost all of a 4G machine's memory, then kill the mmapping applications. The machine instantly goes from 0% of memory dirty to 95% or more. pdflush kicks in and starts writing the least-recently-dirtied pages, which are all highmem. The queue is congested so nobody will write back ZONE_NORMAL pages. kswapd chews 50% of the CPU scanning past dirty ZONE_NORMAL pages and page reclaim efficiency (pages_reclaimed/pages_scanned) falls to 2%. So this patch changes the policy for kswapd. kswapd may use all of a request queue, and is prepared to block on request queues. What will now happen in the above scenario is: 1: The page alloctor scans some pages, fails to reclaim enough memory and takes a nap in blk_congetion_wait(). 2: kswapd() will scan the ZONE_NORMAL LRU and will start writing back pages. (These pages will be rotated to the tail of the inactive list at IO-completion interrupt time). This writeback will saturate the queue with ZONE_NORMAL pages. Conveniently, pdflush will avoid the congested queues. So we end up writing the correct pages. In this test, kswapd CPU utilisation falls from 50% to 2%, page reclaim efficiency rises from 2% to 40% and things are generally a lot happier. The downside is that kswapd may now do a lot less page reclaim, increasing page allocation latency, causing more direct reclaim, increasing lock contention in the VM, etc. But I have not been able to demonstrate that in testing. The other problem is that there is only one kswapd, and there are lots of disks. That is a generic problem - without being able to co-opt user processes we don't have enough threads to keep lots of disks saturated. One fix for this would be to add an additional "really congested" threshold in the request queues, so kswapd can still perform nonblocking writeout. This gives kswapd priority over pdflush while allowing kswapd to feed many disk queues. I doubt if this will be called for. --- include/linux/swap.h | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'include/linux') diff --git a/include/linux/swap.h b/include/linux/swap.h index c635f392d6c1..9ab02098aad5 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -7,6 +7,7 @@ #include #include #include +#include #include #include @@ -14,6 +15,11 @@ #define SWAP_FLAG_PRIO_MASK 0x7fff #define SWAP_FLAG_PRIO_SHIFT 0 +static inline int current_is_kswapd(void) +{ + return current->flags & PF_KSWAPD; +} + /* * MAX_SWAPFILES defines the maximum number of swaptypes: things which can * be swapped to. The swap type and the offset into that swap type are -- cgit v1.2.3