I am debugging an issue with copy_to_user performance on recent linux kernels. I've noticed a 4X slowdown in copy speed when I move from kernel 2.6.18 to more recent kernels on the same hardware. I've tested so far on 2.6.32 and on various 3.x kernels. I noticed that assembly code implementation of copy_to_user has changed, so as a sanity check I ported the 2.6.18 version over to 2.6.32 but saw the same slowdown when using it. This is the hardware I am using:
Dell PowerEdge R710 BIOS version 6.3.0 Intel(R) Xeon(R) CPU x5650 @ 2.67GHz 72GB RAM (ECC DDR3 800MHz) I am running the x86_64 version of course. I see the issue when I do a copy_to_user from a dedicated block of memory located above the 4GB boundary into normal user space mem. The boot parameter mem=65G is passed to the kernel to carve off the dedicated block of memory that is used as a DMA accessible buffer for a specialized network card. That is the buffer we do the copy_to_user from. I should also mention we have been testing this on CentOS 4.8, 6.2, and 6.3, though I've replaced their stock kernels with ones straight from kernel.org, so I don't know that the linux distro is really relevant. I've browsed the kernel mailing list looking for clues and have even been looking at CPU/hardware specific errata, but so far I can see no logical reason why the 2.6.32+ kernels should be performing so much worse than the 2.6.18. Does anyone have any suggestions on areas to dig into besides the copy_to_user implementation itself? Maybe somewhere specific in the MMU code? Any suggestions would be appreciated. Please copy me directly on replies if you don't mind. Thanks, Thad Phetteplace -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
