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Why do I get "vhand spinning" ?

(Contributed by Bela Lubkin):

Well, TA #109419 has a few of the details. Its supposed fix of disabling HPPS is rather peculiar...(*)

The most typical cause of vhand spinning is running out of DMAable memory. OpenServer's kernel memory allocator distinguishes between "DMAable" memory (meaning ISA-DMAable, that is, memory below 16MB) and non-DMAable. In most modern systems, the only device that could possibly need ISA DMAable memory is a floppy drive. Other rarer users would include: ISA sound cards, Adaptec 154x host adapters, and old ISA QIC02/QIC36 tape adapters.

Unfortunately, it's _really_ easy for kernel code to mistakenly request DMAable memory. You have to explicitly request non-DMAable, or else your request is understood to be for DMAable memory. There is a lot of code in the kernel and in third party drivers which mistakenly requests DMAable. This doesn't show up in testing because it's a "no-consequence" bug. Using DMAable memory doesn't hurt the driver at all, it's just a waste.

Except... as overall system memory gets larger over time, people are doing more with systems. The same drivers that used to mistakenly allocate 512K of DMAable may now allocate 1MB, or 2MB... Pretty soon you're completely out of that tiny 16MB window!

Each release of OpenServer has corrected some amount of code that mistakenly requests DMAable. There was a major push to fix these problems in OSR507, and it is almost completely clean of such mistaken requests. (Some are rather hard to root out because multiple drivers use a single memory allocation service that doesn't give a way to specify DMA requirements, and _one_ of those drivers actually does need DMAable, so they all have to accept it.)

When DMAable memory is exhausted and someone requests more of it, vhand starts spinning, looking for memory to use.

As an administrator, you don't have control over most uses of DMAable memory. However, there is one large user that you _do_ control: the buffer cache. The kernel parameter PLOWBUFS controls how many 1K disk buffers are allocated in DMAable address space. You can see how many are currently being used by running `grep bufs /usr/adm/messages` (this might produce a lot of output). On a 507 system that I checked, I get:

kernel: Hz = 100, i/o bufs = 12752k (high bufs = 11728k)

This system has about 13000 I/O buffers, 1024 of which are DMAable (total minus "high"). You want almost all of your buffers to be "high", which are fine for use by PCI host adapters and IDE drives.

PLOWBUFS sets what percentage of total buffers should be allocate below 16MB. If you have 20000 total buffers, setting PLOWBUFS to 1 (its lowest setting) gets you 200 DMAable buffers, which is only 1/80 of the total 16MB space. If you have default parameters for 505, you probably have 6652 total buffers and about 2000 DMAable buffers, so you could save 1.8MB right there. That might be enough to _never_ hit the problem, or might only push the problem horizon out from 48 days to a year or so...

If you have non-default parameters then buffers might be consuming much more of the low 16MB, and you could improve things much more.

Starting with OpenServer 506, the PLOWBUFS parameter has a dual meaning. Values <= 100 mean to allocate that percentage of total buffers from DMAable memory. Values > 100 mean to allocate exactly that many buffers. The machine I was looking at has PLOWBUFS=1024, which is why it got exactly that many DMAable buffers. That's the default setting in 507, chosen to allow floppy drives and old ISA host adapters to work. In a system where the only user is the floppy drive, it could probably be set to the minimum, 101, without negative consequences. Setting it lower than that wouldn't be useful -- if you're going to run out of 15.9MB then you're going to run out of 16.0MB moments later.

Other things that you can change (including PLOWBUFS, so this is a comprehensive remedy list):

- reduce PLOWBUFS to 101 (on 506 or later), or 1 (any release) to sharply decrease the number of DMAable disk buffers

- edit /etc/conf/pack.d/str/space.c, change the value of `str_pool_mem' from MEM_BUF to MEM_KVMAPPED (504 or 505: won't work on 500 or 502; already changed on 506)

- remove NFS from the kernel if you are not using it -- it is a sloppy waster of DMAable memory (fixed in 507)

- if you _are_ using NFS, edit /etc/conf/cf.d/mdevice, find all "nfsd" entries, and make sure they have characteristics 'd' and 'P'. Be careful: the mdevice file cannot be reconstructed from other files. If there is more than one "nfsd" line, they will look different from each other; they're _supposed_ to look different. Just add 'd' and 'P' to the third field of each entry, if they're missing.

- if you're on 504 or earlier and nothing else has worked, disable HPPS

Note that PCI devices can DMA to any address in the system. The word "DMAable" dates back to the ISA days. Reducing DMAable buffers will not harm your modern I/O devices.

<Bela<

(*)Well, not all _that_ peculiar. HPPS was one of those drivers which mistakenly requested DMAable memory. That was fixed starting with 505, so disabling it would not help you if you are on that release.



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