Messages in this thread |  | | | Date | Tue, 23 Mar 2010 18:29:59 -0400 | | From | Rik van Riel <> | | Subject | Re: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone pressure |
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On 03/22/2010 07:50 PM, Mel Gorman wrote:
> Test scenario > ============= > X86-64 machine 1 socket 4 cores > 4 consumer-grade disks connected as RAID-0 - software raid. RAID controller > on-board and a piece of crap, and a decent RAID card could blow > the budget. > Booted mem=256 to ensure it is fully IO-bound and match closer to what > Christian was doing
With that many disks, you can easily have dozens of megabytes of data in flight to the disk at once. That is a major fraction of memory.
In fact, you might have all of the inactive file pages under IO...
> 3. Page reclaim evict-once logic from 56e49d21 hurts really badly > fix title: revertevict > fixed in mainline? no > affects: 2.6.31 to now > > For reasons that are not immediately obvious, the evict-once patches > *really* hurt the time spent on congestion and the number of pages > reclaimed. Rik, I'm afaid I'm punting this to you for explanation > because clearly you tested this for AIM7 and might have some > theories. For the purposes of testing, I just reverted the changes.
The patch helped IO tests with reasonable amounts of memory available, because the VM can cache frequently used data much more effectively.
This comes at the cost of caching less recently accessed use-once data, which should not be an issue since the data is only used once...
> Rik, any theory on evict-once?
No real theories yet, just the observation that your revert appears to be buggy (see below) and the possibility that your test may have all of the inactive file pages under IO...
Can you reproduce the stall if you lower the dirty limits?
> static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan, > struct zone *zone, struct scan_control *sc, int priority) > { > int file = is_file_lru(lru); > > - if (is_active_lru(lru)) { > - if (inactive_list_is_low(zone, sc, file)) > - shrink_active_list(nr_to_scan, zone, sc, priority, file); > + if (lru == LRU_ACTIVE_FILE) { > + shrink_active_list(nr_to_scan, zone, sc, priority, file); > return 0; > }
Your revert is buggy. With this change, anonymous pages will never get deactivated via shrink_list.
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