Re: [squid-users] squid on 32-bit system with PAE and 8GB RAM

Gavin McCullagh wrote:
> Hi,
>
> we're running a reasonably busy squid proxy system here which peaks at
> about 130-150 requests per second.
>
> The OS is Ubuntu Hardy and at the minute, I'm using the packaged 2.6.18
> squid version. I'm considering a hand-compile of 2.7, though it's quite
> nice to get security patches from the distro.

FYI: The latest Intrepid or Jaunty package should work just as well in
Hardy.

>
> We have 2x SATA disks, a 150GB and a 1TB. The linux system is on software
> RAID1 across the two disks. The main cache is 600GB in size on a single
> non-RAID 970GB partition at the end of the 1TB disk. A smaller partition
> is reserved on the other disk as a secondary cache, but that's not in use
> yet and the squid logs are currently written there. The filesystems for
> the caches are reiserfs v3 and the cache format is AUFS.
>
> We've been monitoring the hit rates, cpu usage, etc. using munin. We
> average about 13% byte hit rate. Iowait is now a big issue -- perhaps not
> surprisingly. I had 4GB RAM in the server and PAE turned on. I upped this
> to 8GB with the idea of expanding squid's RAM cache. Of course, I forgot
> that the squid process can't address anything like that much RAM on a
> 32-bit system. I think the limit is about 3GB, right?

For 32-bit I think it is yes. You can rebuild squid as 64-bit or check
the distro for a 64-bit build.

However keep this in mind: rule-of-thumb is 10MB index per GB of cache.

So your 600 GB disk cache is likely to use ~6GB of RAM for index +
whatever cache_mem you allocate for RAM-cache + index for RAM-cache + OS
and application memory.

>
> I have two questions. Whenever I up the cache_mem beyond about 2GB, I
> notice squid terminates with signal 6 and restarts as the cache_mem fills.
> I presume this is squid hitting the 3GB-odd limit? Could squid not behave
> a little more politely in this situation -- either not attempting to
> allocate the extra RAM, giving a warning or an error?

cache.log should contain a FATAL: message and possibly a line or two
beforehand about why and where the crash occured.
Please can you post that info here.

>
> My main question is, is there a sensible way for me to use the extra RAM?
> I know the OS does disk caching with it but with a 600GB cache, I doubt
> that'll be much help.

RAM swapping (disk caching by the OS) is one major performance killer.
Squid needs direct access to all its memory for fast index searches and
in-transit processing.

> I thought of creating a 3-4GB ramdisk and using it
> as a volatile cache for squid which gets re-created (either by squid -z or
> by dd of an fs image) each time the machine reboots. The things is, I
> don't know how squid addresses multiple caches. If one cache is _much_
> faster but smaller than the other, can squid prioritise using it for the
> most regularly hit data or does it simply treat each cache as equal? Are
> there docs on these sorts of issues?

No need that is already built into Squid. cache_mem defines the amount
of RAM-cache Squid uses.

Squid allocates the disk space based on free space and attempts to
spread the load evenly over all dirs to minimize disk access/seek times.
cache_mem is used for the hottest objects to minimize delays even further.

Amos

-- 
Please be using
   Current Stable Squid 2.7.STABLE6 or 3.0.STABLE13
   Current Beta Squid 3.1.0.6