linux-kernel - huge mem mmap eats all CPU when multiple processes

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Date:	Mon, 8 Jun 2009 10:27:49 -0400
From:	Matthew Von Maszewski <matthew@...hive.com>
To:	linux-kernel@...r.kernel.org
Subject: huge mem mmap eats all CPU when multiple processes 

[note: not on kernel mailing list, please cc author]

Symptom:  9 processes mmap same 2 Gig memory section for a shared C  
heap (lots of random access).  All process begin extreme CPU load in  
top.

- Same code works well when only single process access huge mem.
- Code works well with standard vm based mmap file and 9 processes.

Environment:

- Intel x86_64:  Dual core Xeon with hyperthreading (4 logical  
processors)
- 6 Gig ram, 2.5G allocated to huge mem
- tried with kernels 2.6.29.4 and 2.6.30-rc8
- following mmap() call has base address as NULL on first process,  
then returned address passed to subsequent processes (not threads,  
processes)

            m_MemSize=((m_MemSize/(2048*1024))+1)*2048*1024;
             m_BaseAddr=mmap(m_File->GetFixedBase(), m_MemSize,
                             (PROT_READ | PROT_WRITE),
                             MAP_SHARED, m_File->GetFileId(), m_Offset);


I am not a kernel hacker so I have not attempted to debug.  Will be  
able to spend time on a sample program for sharing later today or  
tomorrow.  Sending this note now in case this is already known.

Don't suppose this is as simple as a Copy-On-Write flag being set wrong?

Please send notes as to things I need to capture to better describe  
this bug.  Happy to do the work.

Thanks,
Matthew
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