On Thu, 5 Apr 2007, Luck, Tony wrote:
> > This implements granule page sized vmemmap support for IA64.
>
> Christoph,
>
> Your calculations here are all based on a granule size of 16M, but
> it is possible to configure 64M granules.
Hmm...... Maybe we need to have a separate size for the vmemmap size?
> With current sizeof(struct page) == 56, a 16M page will hold enough
> page structures for about 4.5G of physical space (assuming 16K pages),
> so a 64M page would cover 18G.
Yes that is far too much.
> Maybe a granule is not the right unit of allocation ... perhaps 4M
> would work better (4M/56 ~= 75000 pages ~= 1.1G)? But if this is
> too small, then a hard-coded 16M would be better than a granule,
> because 64M is (IMHO) too big.
I have some measurements 1M vs. 16M that I took last year when I first
developed the approach:
1. 16k vmm page size
Tasks jobs/min jti jobs/min/task real cpu
1 2434.08 100 2434.0771 2.46 0.02 Thu Oct 12 03:22:20 2006
100 178784.27 93 1787.8427 3.36 7.14 Thu Oct 12 03:22:34 2006
200 279199.63 94 1395.9981 4.30 14.70 Thu Oct 12 03:22:52 2006
300 340909.09 92 1136.3636 5.28 22.55 Thu Oct 12 03:23:14 2006
400 381133.87 90 952.8347 6.30 30.64 Thu Oct 12 03:23:40 2006
500 408942.20 93 817.8844 7.34 38.90 Thu Oct 12 03:24:10 2006
600 430673.53 89 717.7892 8.36 47.15 Thu Oct 12 03:24:45 2006
700 445859.87 92 636.9427 9.42 55.59 Thu Oct 12 03:25:23 2006
800 460564.19 94 575.7052 10.42 63.57 Thu Oct 12 03:26:06 2006
2. 1M vmm page size
Tasks jobs/min jti jobs/min/task real cpu
1 2435.06 100 2435.0649 2.46 0.02 Thu Oct 12 03:08:25 2006
100 178041.54 93 1780.4154 3.37 7.18 Thu Oct 12 03:08:39 2006
200 278035.22 96 1390.1761 4.32 14.85 Thu Oct 12 03:08:57 2006
300 338536.77 96 1128.4559 5.32 22.90 Thu Oct 12 03:09:19 2006
400 377180.58 89 942.9514 6.36 31.19 Thu Oct 12 03:09:46 2006
500 407000.41 96 814.0008 7.37 39.21 Thu Oct 12 03:10:16 2006
600 428979.98 91 714.9666 8.39 47.43 Thu Oct 12 03:10:51 2006
700 444209.41 94 634.5849 9.46 55.86 Thu Oct 12 03:11:30 2006
800 455753.89 93 569.6924 10.53 64.59 Thu Oct 12 03:12:13 2006
4M would be right in the middle and maybe not so bad.
Note that these numbers were based on a more complex TLB handler.
See http://marc.info/?l=linux-ia64&m=116069969308257&w=2 (variable
kernel page size handler).
The problem with a different page size is that this would require
redesign of the TLB lookup logic. We could go back to my variable kernel
page size patch quoted above but then we walk the complete page table.
The 1 level lookup as far as I can tell only works well with 16M.
If we would try to use a 1 level lookup for a 4M page then we would have
a linear lookup table that takes up 4MB to support 1 Petabyte.
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