Re: [PATCH] mm/ksm : Checksum calculation function change (jhash2 -> crc32)

[Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>]

On Tue, 1 Aug 2017 22:05:50 +0200
Andrea Arcangeli <aarcange@redhat.com> wrote:

> On Tue, Aug 01, 2017 at 09:07:35PM +0900, leesioh wrote:
> > In ksm, the checksum values are used to check changes in page
> > content and keep the unstable tree more stable. KSM implements
> > checksum calculation with jhash2 hash function. However, because
> > jhash2 is implemented in software, it consumes high CPU cycles
> > (about 26%, according to KSM thread profiling results)
> > 
> > To reduce CPU consumption, this commit applies the crc32 hash
> > function which is included in the SSE4.2 CPU instruction set.
> > This can significantly reduce the page checksum overhead as follows.
> > 
> > I measured checksum computation 300 times to see how fast crc32 is
> > compared to jhash2. With jhash2, the average checksum calculation
> > time is about 3460ns, and with crc32, the average checksum
> > calculation time is 888ns. This is about 74% less than jhash2.  
> 
> crc32 may create more false positives than jhash2. crc32 only
> guarantees a different value in return if fewer than N bit
> changes. False positives in crc32 comparison, would result in more
> unstable pages being added to the unstable tree, and if they're
> changing as result of false positives it may make the unstable tree
> more unstable leading to missed merges (in addition to the overhead of
> adding those to the unstable tree in the first place and in addition
> of risking an immediate cow post merge which would slowdown apps even
> more).
> 
> I think if somebody wants a crc instead of a more proper hash (that is
> less likely to generate false positives if a couple of bits changes)
> it should be an option in sysfs not enabled by default, but overall I
> think it's not worth this change for a downgrade to crc. There's the
> risk an admin thinks it's going to make things runs faster because KSM
> CPU utilization decreases, but missing the risk of increased CoWs in
> app context or missed merges because of higher instability in the
> unstable tree.

that's true, but it's possible that all the extra work due to
additional collisions could still be less than the time saved with a
faster checksum. Also, even within the same architecture, different
checksums can have different performances depending on CPU vendor and
model. I would still let the admin (or ksmtuned) choose.

> Still deploying hardware accelleration in the KSM hash is a
> interesting idea that I don't recall has been tried. Could you try to
> benchmark in userland (or kernel if you wish) software jhash2 vs
> CONFIG_CRYPTO_SHA1_SSSE3 or CONFIG_CRYPTO_GHASH_CLMUL_NI_INTEL instead
> of the accellerated crc?  (I don't know if GHASH API can fit our use
> case though, but accellerated SHA1 sure would fit).  I suppose they'll
> be slower than crc32, and probably slower than jhash2 too, however I
> can't be sure by just thinking about it.
>
> We've to also keep the floating point save and restore into account in
> the real world, where ksm schedules often and may run interleaved in
> the same CPU where an app uses the fpu a lot in userland (if the
> interleaved app doesn't use the fpu in userland it won't create
> overhead).

that is also true, although some CPUs have basic (and not-so-basic)
crypto functions baked in, so no need to save or restore FPU registers
in those cases. 


best regards

Claudio

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