Re: [squid-users] shared memory seems to allow size of 32K **1KB** segments (32MB)...

From: Marcus Kool <>
Date: Tue, 07 Aug 2012 12:25:45 -0300

Amos referred to a mail thread where the issues of the 32K limit were discussed.
This thread does not suggest that 32K is a hard shared memory system limit
but merely that 32K is a Squid-specific design limitation.

Oracle and other RDBMS use very large shared memory segments of
multiple gigabytes. 32MB for SHMMAX is a default on some
systems but can be altered and set much higher.

The implication of the design decisions
- use one page of 32K for shared objects
- use multi-process architecture vs multi-thread architecture with a shared memory segment
imply the current limit that the maximum size for a shared object
is 32K and that larger objects cannot be shared between worker processes
using the shared memory segment. I think that everybody involved
does not like the limitation. Observing the conversations
in the squid-dev list it seems there is not sufficient time/funding to
do everything at this time.


On 08/05/2012 07:37 PM, Linda W wrote:
> Linda W wrote:
>> Amos Jeffries wrote:
>>> We are still limited to one page,
>> ---
>> 1 page or 1 segment/item?
> ----
> I don't know who 'we' is... but on x86_64 linux, I was able to use the
> perl SysV::IPC calls shmget/shmwrite/shmread/shmctl to allocate up to
> my system's run-time limit (which I can up if needed) of 32MB/shm segment.
> It looks like there is an underlying granularity of 8KB, but shouldn't
> it be easy to simply use the shm interface and allocate exact size segments
> to hold shared files?
> Hmmm......
> Either that or allocate in largest size chunksize available and sub-divide it.
> As for disk -- if the index of files that were stored on disk -- why
> couldn't the processes share a file cache? Certainly you don't want
> two separate processes downloading the same file at the same time -- that
> would really hurt bandwidth...
Received on Tue Aug 07 2012 - 15:25:48 MDT

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