"Checksumming speed". What is that?

IceRAM · 2004-05-11 13:36:57

Kernel 2.6.5:

kernel: md: linear personality registered as nr 1
kernel: md: raid0 personality registered as nr 2
kernel: md: raid1 personality registered as nr 3
kernel: md: raid5 personality registered as nr 4
kernel: raid5: measuring checksumming speed
kernel:    8regs     :   824.000 MB/sec
kernel:    8regs_prefetch:   652.000 MB/sec
kernel:    32regs    :   368.000 MB/sec
kernel:    32regs_prefetch:   384.000 MB/sec
kernel:    pIII_sse  :   900.000 MB/sec
kernel:    pII_mmx   :  1124.000 MB/sec
kernel:    p5_mmx    :  1200.000 MB/sec
kernel: raid5: using function: pIII_sse (900.000 MB/sec)

Kernel 2.6.6:

kernel: md: linear personality registered as nr 1
kernel: md: raid0 personality registered as nr 2
kernel: md: raid1 personality registered as nr 3
kernel: md: raid5 personality registered as nr 4
kernel: raid5: measuring checksumming speed
kernel:    8regs     :   824.000 MB/sec
kernel:    8regs_prefetch:   652.000 MB/sec
kernel:    32regs    :   368.000 MB/sec
kernel:    32regs_prefetch:   384.000 MB/sec
kernel:    pIII_sse  :   880.000 MB/sec
kernel:    pII_mmx   :  1120.000 MB/sec
kernel:    p5_mmx    :  1196.000 MB/sec
kernel: raid5: using function: pIII_sse (880.000 MB/sec)

Questions:
1. Why did the speed decrease in 2.6.6?
2. If it's a speed, shouldn't the highest be chosen (p5_mmx in this case - I have a pIII)
3. What is it used for? (taking into account I don't have RAID hdds)

P.S.
1. When changing from 2.6.4 to 2.6.5 I've noticed the increase in speed.
2. I did google it (this question)
3. I know this thing might be minor (or not important), but I do like to understand my kernel.

xerxes2 · 2004-05-11 18:23:51

Hallo IceRam.
This is newbiecorner, maybe you should try kernelhackers mailinglist instead.

IceRAM · 2004-05-11 18:53:49

xerxes2 wrote:

Hallo IceRAM.
This is newbiecorner, maybe you should try kernelhackers mailinglist instead.

Hmm... I know that there are specialized lists for everything (especially for linux), but I thought this was a good place to ask instead of subscribing everywhere for one question.
I understand most of the linux kernel messages. They are only informative, but when they display a method of thinking (like choosing the checksumming method) I'm starting to ask myself the same thing. The others are simply "Hey, you've got XYZ".. and I say: "wow.. I have XYZ, it's great that you found it..." or "I didn't even know that it knows that..."... etc. The linux kernel makes you learn lots of things on your computer.

P.S. I know this is the newbie corner, maybe newbies want also to understand the kernel... not only applications, libraries, linux programming, package management, memory management etc. Is that a problem?
(ok.. maybe that's not really a newbie)
P.S. 2. I've used RH, MDK, SuSE before but I don't think I've ever asked myself so much about the kernel. Maybe it's an Arch thing.. the will of finding more... who knows...

skoal · 2004-05-12 03:53:06

IceRAM wrote:

Questions:
1. Why did the speed decrease in 2.6.6?
2. If it's a speed, shouldn't the highest be chosen (p5_mmx in this case - I have a pIII)
3. What is it used for? (taking into account I don't have RAID hdds)
...

As you probably know by now, the checksumming speed the Raid 5 module chooses has to do with parity checking across your disk array.

In other words, if you have 10 disks, and you want to write out a block, the RAID system reads the pieces from each disk, calculates parity, writes the block, and updates the disk doing parity for that block. That's basically the checksum algorithm, I believe.

As far as choosing a "slower" checksum, the RAID 5 module will pick the one for your architecture only. I believe this has to do with the developers using different checksum algorithms for each architecture type, instead of having 1 big algorithm that might not work as well across all processors versus having 1 specific for each.

The difference in speed (higher or lower) from kernel revisions, might be simply due to additions or deletions in the checksum algorithm for that kernel revision. It's kind of like removing or adding code from your own program and making it run faster or slower. But, "slower" may have better overall performance and reliability. Those numbers are just benchmarks.

That's about the extent I know. And, I haven't used a RAID array in some time.

IceRAM · 2004-05-12 05:46:20

10x...

xerxes2 · 2004-05-12 22:16:30

P.S. 2. I've used RH, MDK, SuSE before but I don't think I've ever asked myself so much about the kernel. Maybe it's an Arch thing.. the will of finding more... who knows...

Exactly the same happened to me...

Arch Linux

#1 2004-05-11 13:36:57

"Checksumming speed". What is that?

#2 2004-05-11 18:23:51

Re: "Checksumming speed". What is that?

#3 2004-05-11 18:53:49

Re: "Checksumming speed". What is that?

#4 2004-05-12 03:53:06

Re: "Checksumming speed". What is that?

#5 2004-05-12 05:46:20

Re: "Checksumming speed". What is that?

#6 2004-05-12 22:16:30

Re: "Checksumming speed". What is that?

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