 | | | No swap being used | No swap being used 2006-01-12 - By Jeff Macfarland
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William Warren wrote:
> It depends on kernel tuning but most times the kernel will start dumping
> the file cache to free up ram. Once that is done it will start
> swapping. You pretty much have to chew up all ram before swap stats
> getting used. If you are having high iowaits then your issue is hdd or
> IO not swapping. Check your RAID controller or you may just need to
> upgrade to faster disks.
We're pretty sure its IO as well but are keeping our eyes and ears open.
We have a case open with Sun about the slowdowns (sunrays are severely
affected, but ssh prompts seize up briefly as well)
The 14 disks in question are 15k rpm 36G FC drives in an old compaq
enclosure. Its an older loop device with mdadm handling raid.
I remember back in the day when 2.4.2x kernels were new where I had to
use some of Ingo Molnar's (sp?) latency patches or else desktop usage
was pitiful if the machine was doing any sort of IO. I've asked about
this IO problem before on this list (and elsewhere) and go no replies so
I figured it was just us experiencing this issue and not for lack of
kernel latency patches.
IIRC, however, iostat shows high await times on /dev/sda whereas the cvs
repo is on the fc enclosure (/export) by way of a symlink on /.
>
> Jeff Macfarland wrote:
>> Jussi Silvennoinen wrote:
>>>> Sun V20z, RHEL 3u5, i386 kernel, 12GB RAM, 12GB swap
>>>>
>>>> This box hosts quite a few things, among them serving SunRay's. We've
>>>> been experiencing horrible slowdowns particularly during a cvs
>>>> checkout.
>>>> Just noticed that NO swap is being used:
>>>>
>>>> output of `free`:
>>>>
>>>> total used free shared buffers cached
>>>> Mem: 12277844 12036764 241080 0 1333552 7502204
>>>> -/+ buffers/cache: 3201008 9076836
>>>> Swap: 12578704 0 12578704
>>>
>>> I guess yer a Solaris-admin? Things work a bit different in the
>>> penguin-world.
>>>
>>> No need to swap = nothing gets swapped. Solaris does it a bit different.
>>> Over 7gb's of your ram is used for block cache so you're far away
>>> from actually needing to swap.
>>>
>>
>> At what point will linux start using swap? Where there is absolutely
>> no free RAM left? Its been my understanding that swap will almost
>> always be used for idle processes regardless of amount of free ram
>> left. I see the output of free range from a few gigs free to 50M and
>> at no point is anything swapped. I suppose I am just used to our old
>> solaris boxes though.
>>
>>> Instead, look at the output of 'top' when the slowdowns occur.
>>
>> We have. IO waits perecentage goes quite high and iostat shows 'await'
>> time on the order of a few thousand ms for sda. The cvs repo is on a
>> software raid partition though.
>>
>> Thanks for the swap info though.
>>
>
--
Jeff Macfarland (jmacfarland@(protected))
Nexa Technologies - 972.747.8879
Unix Administrator
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