High mem needed in kernel?

[mensan]

hey there all,

i have looked around and cant figure this out. i was talking to an op in a #gentoo irc channel who seemed to know what he was talking about with the 2.6.X line of kernels, but he kept telling me that in this box i am running with 1.5GB of ram, i didnt need high mem set to 4GB in the kernel.

without it set to 4GB, i am limited to gentoo seeing only 1GB, with the 4GB setting i get all 1.5GB. i told him this and he denied it. i asked if there was any harm in setting it to 4GB and he said that i could see performance hits.

anyway, i have it set to 4GB now, should i need this? does this drop performance at all? if i turn it off, is there something else i should set in teh kernel to make the rest of my ram usable? what is the bottom line on that high mem stuff?

thanks,
men$an

[Halcy0n]

[mensan]

well thanks, i will have to present this as evidence. also, anyone know anything about the performance hit he was talking about?

[Raoul_Duke]

Not heard about a peformance hit....my second box has highmem enabled and i've never noticed anything
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[kashani]

I believe there were some oddities when highmem was first added. There is some overhead to the kernel to create the 1G/3G split, etc which might be where he was getting his info. I'm reasonable sure all that was solved pretty early in 2.4.x. This page might shed some light on the overhead of large RAM system. Not as bad as having a mere 2-4GB, but interesting none the less.

http://lwn.net/Articles/39283/

kashani
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Will personally fix your server in exchange for motorcycle related shop tools in good shape.

[mensan]

thanks for the clarification, and yes, Giordano's is GREAT pizza up here in teh chicago area kashani. ;D you should try Lou Malnati's too though. ;P