On Sun, Sep 04, 2011 at 06:19:16PM -0400, Martin K. Petersen wrote: > >>>>> "Andi" == Andi Kleen <andi@xxxxxxxxxxxxxx> writes: > > Andi> Doesn't have any performance numbers. > > It's been a while since I read them. I thought they had some compelling > numbers. Anyway, made a big difference in real life testing here. For > sustained I/O we're talking an order of magnitude. > > > Andi> You need to keep in mind that PCLMULQDQ uses FPU state, so any > Andi> speedup for the kernel must be large enough to amortize the cost > Andi> of saving the FPU state. > > Yeah, my test cases were for bulk database I/O, not for writing a > handful of fs metadata blocks. Plus for the DB tests the CRC was > generated in userland. > > I seem to recall Joel picking something other than the hw-accelerated > CRC32C for ocfs2 metadata and that didn't cause any problems. Yes, he picked regular CRC32, which has a reasonably fast slice-by-4 software implementation. For ext4, my original choices were hw acceleration or the slower single-byte lookup table. With hw acceleration the overhead of adding the checksums is about ~10% (for just the metadata operations); with the single-byte table it was about 50%; and with the proposed slice-by-8 patch it's about 20%. Hopefully I can optimize this even more in the future. > That said, I do see a difference between IP checksum and CRC on normal > FS workloads with DIX enabled here. I would hope so, since the IP checksum is much simpler than any CRC... --D > Andi> Typically that only works out for quite large buffers, but kernel > Andi> buffers are relatively small. > > *nod* > > -- > Martin K. Petersen Oracle Linux Engineering > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
