Strange memcached behavior on stress test

Janning Vygen vygen at
Tue Jun 13 10:00:53 UTC 2006

Thank you very much!
There is nothing more to say. Great post! I fully understood it now and i will 
take a look at sharedance.

kind regards 

Am Dienstag, 13. Juni 2006 11:06 schrieb Garth Webb:
> On Tue, 2006-06-13 at 09:39 +0200, Janning Vygen wrote:
> > Am Montag, 12. Juni 2006 18:17 schrieb Brad Fitzpatrick:
> > > On Mon, 12 Jun 2006, Janning Vygen wrote:
> > > > > (though that depends on your usage pattern) but this is one
> > > > > condition where you'll get a set problem and an out of memory error
> > > > > even if you have space left in individual slabs.
> > > >
> > > > Thank you garth for this detailed explanation. Now i think i
> > > > understood my problem. My session data is very different in size. So
> > > > my allocated "slabs" are "full" and a new slab can't be allocated.
> > >
> > > Janning,
> > >
> > > This problem is fixed in two different ways in later releases.  Try out
> > > this version for now:
> > >
> > >
> > >
> > > It pre-allocates a 1MB slab of each class on start, so people won't run
> > > into this confusion.
> >
> > Ok, i would run a newer version of memcached IF memcached is the right
> > thing for (PHP) session management. I am not sure about it anymore. Some
> > people on the list told me it is not the right thing because it saves
> > it's session in memory. But i dont think that this is a problem. But i
> > think session management with memcached can run into problems if you have
> > a peak with lots of pages as the oldest session data is moved out and you
> > do not have a garanteed session time anymore.
> >
> > If i have a memcached instance with 2GB memory and my average session
> > data is 8 KByte (slab size) i can save 262144 items. As the oldest value
> > is pushed out, i will never have a guranteed session expiration time (in
> > terms as you are still logged in after 20 minutes of inactivity). A
> > memory is quite expensive it maybe does not make sense to use memcached
> > as a session storage.
> Exactly!  Memcached is *not* storage.  Memcached is meant to make your
> *existing* storage solution (disk/mysql/etc) faster.  Let me outline a
> situation.  It is very unlikely that all of your users will be logged in
> at the same time.  Lets say you have 300,000 users.  I don't know what
> your application is, but on most web apps, only a small percentage of
> users are logged in at any given time.  Over any 20 minute period, maybe
> there are 2000 users actively logged in.  You said that session is about
> 8K, this means that *you only need a memcached size of 16MB*.
> Your session data can be stored in MySQL or SQL-lite or flatfiles or
> whatever, it doesn't matter.  Why?  Because you are only going to access
> that data *once* and load it into memcached.  At that point you will
> access it from memcached.  The cost of loading that data from slow
> storage will be unnoticeable because it only happens the first time it
> is requested.  If the user continues to stay online, their data will
> continue to stay in memcached because memcached keeps active data in the
> cached.  If they leave and do something else for a while their data will
> eventually be pushed out by other content.
> > An i am now convinced that sharedance might do it much better, because it
> > uses disks to store session data.
> Sharedance does not simply store data to disk.  It caches data in memory
> just like memcached and THEN uses a file on disk for permanent storage.
> Memcached will behave *exactly* like sharedance by adding the behavior I
> mentioned above, ie:
> 1) Look for data in the cache
> 2) If its in the cache return it, if its not goto 3
> 3) Look on storage (disk, mysql, etc)
> 4) Write the data found to the cache
> 5) Return the data
> With this algorithm, data is only loaded from slow storage once and then
> kept in cache while its in use.  If the cache goes down, or data gets
> pushed out it is transparent to the user because the system will always
> fall back to the slow storage.  In the case of total cache failure, the
> site maybe slower, but it will still continue to function.
> Memcached is a very fast caching solution.  It can be used for steps 1
> and 2 of the above process.  You must write code to work with your
> permanent storage in steps 3, 4 and 5.  Its advantage is that it
> integrates with your existing storage system.
> Sharedance is a caching solution that ALSO implements a permanent
> storage solution.  It does steps 1-5.  Its advantage is that you don't
> have to choose a storage system.
> So it sounds like you want sharedance because you don't want to do any
> extra work to and you do not currently store data anywhere else (mysql,
> disk, etc).
> Garth
> > Maybe i should take a look at sharedance which was always on my option
> > list. But memached rocks at the moment (beside the memory problem i
> > encountered once). And i feel that memcached is more used and better
> > supported.
> >
> > kind regards,
> > janning

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