Messages in this thread | | | From | Gerrit Huizenga <> | Subject | Re: High Mem Options | Date | Wed, 05 Mar 2003 12:31:03 -0800 |
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On Wed, 05 Mar 2003 04:26:54 PST, William Lee Irwin III wrote: > On Wed, Mar 05, 2003 at 06:28:36AM -0500, Reed, Timothy A wrote: > > Yet another quick question...is there any down side to using the > > 64GB option over the 4GB option if the machine only has 2GB of RAM onboard?? > > I would think this would be a performance issue? Does the kernel only use > > the translation table if it has to access any memory location over 4GB? > > Yes, the additional level of pagetables slows things down quite a bit.
Bill, do you hvae measures for this? I seem to remember PTX's impact of PAE36 as being about 3-5% depending on workload. Janet did one test sometime back with DB2 that showed a net of no difference on TPC-H (PAE slows things down, less memory pressure speeds things up) but Badari just repeated with 2.5.62 or 2.5.63 and saw a larger degradation.
I'm wondering if some hardware is not getting correctly configured at boot with with respect to MTRR's, perhaps... I really wouldn't expect a 10% impact from PAE and I don't have any consistent Linux measurements to validate or invalidate that much impact.
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