Cached_model and memcache-client slowness


#1

Hi there,

I’ve got a Rails app that’s a little unusual in that it maintains a
database representation of a filesystem (for an image gallery). Thus, if
the filesystem is essentially ‘out of date’ then the controller’s list
action will end up invoking a model update (which involves a filesystem
scan).

I’m using sqlite3 as this app will be deployed in an environment where
having a standalone app, easily launchable via Locomotive, will be
necessary, but the server will not be under really high load.

Anyways, to try to speed up the database reads I made my models
subclasses of cached_model, and set up memcached. It’s clearly working,
as I see the gets and puts in my log. However, cached_model and
memcached have if anything slowed everything down. I’m still seeing SQL
queries that seem extraneous.

For example:

Rendering gallery/browse
MemCache Get (0.182419) active_record:Entry:6
Entry Load (0.003009) SELECT * FROM entries WHERE (entries.id = 6)
LIMIT 1
MemCache Set (0.388076) active_record:Folder:6
Entry Load (0.002979) SELECT * FROM entries WHERE (entries.id = 1)
LIMIT 1
MemCache Set (0.377813) active_record:Folder:1
MemCache Set (0.388034) active_record:Folder:1
MemCache Set (0.391366) active_record:Folder:1
Rendered gallery/_navbar (1.79293)
Rendered gallery/_thumbs (0.03502)
Rendered gallery/_view_footer (0.01790)
MemCache Get (0.175059) active_record:Entry:6
Entry Load (0.002988) SELECT * FROM entries WHERE (entries.id = 6)
LIMIT 1
MemCache Set (0.346483) active_record:Folder:6

I’m not sure why the extra SELECT for entry id 6.
And even beside that, it just seems extremely slow compared with not
using memcached. Why would this be? This is on Mac OS X 10.4.5, using
Ruby 1.8.4, Rails 1.0, SWIG 1.3.27 for the sqlite3-ruby bindings,
sqlite3 3.1.3, lighttpd 1.4.8 with fcgi, etc.

thanks,

Noah


#2

Ok, so I did a bit of googling and found this entry on the memcached
mailing list:
http://lists.danga.com/pipermail/memcached/2006-January/001898.html

So it looks like memcached on OS X may be at fault. I notice that Eric
Hodel mentions on the Segment7 blog: “You can now use cached_model
without memcache, but you wonâ??t necessarily see the same speed benefits.
(By default OS X has a horribly slow memcached so I want to avoid it on
OS X.)”

Ok… is there any point to using cached_model without memcached? Is it
that you use the local cache instead? (the in-process cache).

thanks…


#3

Ok, a bit further along… I can’t seem to get cached_model to work
without memcached, even though the docs say it’s possible.

If I don’t create the MemCache instance called CACHE in environment.rb,
then (even with CachedModel.use_local_cache = true and
CachedModel.use_memcache = false) it complains about uninitialized
constant CACHE (because CACHE doesn’t exist as an instance of MemCache)
in memcache_util.rb.

If I do create the MemCache instance, I see it trying to use memcache
(and thus being horribly slow because memcached is not usable on OS X…
c’mon apple, fix this kqueue() bug already).

So how to get cached_model to work using just the in-process (local)
cache?

And how to get apple to fix this kernel bug (if it still exists… is it
possible that it has been fixed and this workaround in libevent is no
longer needed?)


#4

Eric H. wrote:

Are you sure its not TCP_NOPUSH?

It is TCP_NOPUSH and kqueue. Both are broken. Turns out there’s a fix
to get memcached to be fast on OS X, but it will not perform well under
very high load (high in terms of concurrency). Set the EVENT_NOKQUEUE
environment variable to 1 before running memcached, and undef TCP_NOPUSH
in memcached.c and rebuild memcached.


#5

On Mar 2, 2006, at 6:32 AM, Noah wrote:

If I do create the MemCache instance, I see it trying to use memcache
(and thus being horribly slow because memcached is not usable on OS
X…
c’mon apple, fix this kqueue() bug already).

So how to get cached_model to work using just the in-process (local)
cache?

I should get a round-tuit to fix this bug (and one other) this week.
My bad.

And how to get apple to fix this kernel bug (if it still exists…
is it
possible that it has been fixed and this workaround in libevent is no
longer needed?)

Report it to apple. I’m not sure where the bug-report form is, though.

Are you sure its not TCP_NOPUSH?


Eric H. - removed_email_address@domain.invalid - http://blog.segment7.net
This implementation is HODEL-HASH-9600 compliant

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