linux-kernel - Re: [PATCH 1/3] page-allocator: Under memory pressure, wait on pressure to relieve instead of congestion

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Date:	Tue, 09 Mar 2010 16:56:03 +0100
From:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	Mel Gorman <mel@....ul.ie>, linux-mm@...ck.org,
	Nick Piggin <npiggin@...e.de>,
	Chris Mason <chris.mason@...cle.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] page-allocator: Under memory pressure, wait on pressure
 to relieve instead of congestion



Christoph Lameter wrote:
> On Mon, 8 Mar 2010, Mel Gorman wrote:
> 
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index 30fe668..72465c1 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -398,6 +398,9 @@ struct zone {
>>  	unsigned long		wait_table_hash_nr_entries;
>>  	unsigned long		wait_table_bits;
>>
>> +	/* queue for processes waiting for pressure to relieve */
>> +	wait_queue_head_t	*pressure_wq;
>> +
>>  	/*
> 
> The waitqueue is in a zone? But allocation occurs by scanning a
> list of possible zones.
> 
>> +long zonepressure_wait(struct zone *zone, unsigned int order, long timeout)
> 
> So zone specific.
> 
>> -		if (!page && gfp_mask & __GFP_NOFAIL)
>> -			congestion_wait(BLK_RW_ASYNC, HZ/50);
>> +		if (!page && gfp_mask & __GFP_NOFAIL) {
>> +			/* If still failing, wait for pressure on zone to relieve */
>> +			zonepressure_wait(preferred_zone, order, HZ/50);
> 
> The first zone is special therefore...
> 
> What happens if memory becomes available in another zone? Lets say we are
> waiting on HIGHMEM and memory in ZONE_NORMAL becomes available?

Do you mean the same as Nick asked or another aspect of it?
citation:
"I mean the other way around. If that zone's watermarks are not met, 
then why shouldn't it be woken up by other zones reaching their watermarks."


-- 

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
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