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KernelTrap: Page Clustering, Booting Linux On A 64GB x86

Apr 04, 2003, 10:00 (0 Talkback[s])
(Other stories by Jeremy Andrews)

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"William Lee Irwin III recently announced on the lkml that he'd successfully gotten Linux running on a 64GB x86 server. His posts included two different boot message logs, one without his page clustering patch, and one with. In the latter case, his patch overcomes the 1GB mem_map virtual space limitation imposed by x86 32-bit servers, without which the kernel over-runs allowable memory space.

"Bill's current efforts are based upon Hugh Dicken's earlier page clustering patches for the 2.4.x kernel. Hugh's efforts were actually focused on allowing larger filesystem block sizes, prompting Bill to say, 'The fact it resolves the horror of mem_map[] overrunning kernel virtualspace on i386 PAE is really an obscure coincidence.' His patch is still a work in progress, but with time will offer a number of additional benefits beyond the support of 64GB x86 servers. For example, utilizing the entire software page in fault handlers results in prefaulting benefits, and increasing the physical contiguity of data results in I/O throughput benefits. However, at this time 'until it is done it will have severe performance problems on small memory machines (say, less than 16GB).'

"I approached Bill, asking questions to better understand what he was working toward. He replied with a wealth of information, including several ASCII diagrams and lengthy explanations. To summarize, he offered: 'Without pgcl, 64GB is a doorstop, because in /proc/meminfo LowTotal: was a mere 176MB and so incapable of supporting any significant loads. With pgcl, 64GB functions quite nicely, because LowTotal is 750MB and has room for all the kernel bloat that should be there (but things that shouldn't still need to be fixed)...'"

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