[Sbcl-help] Garbage collection for server with user sessions

[Brett v. de S. <bv...@as...>]

I am using sbcl for a web-server application (http://github.com/bvds/andes)
for an intelligent tutor homework system.  Each instance of a student
solving a problem has a user session associated with it.  Each session 
lasts for roughly 15 minutes (but could vary quite a bit) and uses 
roughly 1MByte of memory (on a 32 bit machine).   When the 
server is fully loaded, it can handle about 300 concurrent sessions 
without maxing out the cpu.

However, I am having lots of trouble with garbage collection, since 
the behaviour of the server badly violates the basic assumption of 
the generational garbage collector:  in my case, it is more likely that 
old data (after a session has closed) can be garbage collected 
than newer data.

For simultaneous 300 sessions, the memory usages looks like this:
Breakdown for dynamic space:
  274,297,608 bytes for 34,287,201 cons objects.
  95,976,736 bytes for 1,372,664 simple-character-string objects.
  37,470,400 bytes for 1,170,950 symbol objects.
  45,860,504 bytes for 1,064,281 other objects.
  453,605,248 bytes for 37,895,096 dynamic objects (space total.)


Any advice for what to do?  Two ideas:

1.  Modify the garbage collector so that it gcs the older data first.
(that is, do generation 0, 1, 2 in the usual way, but do generations 
3, 4, 5 backwards).

2.  Is there any way to garbage collect for a specific object
and its descendents?

(For the purposes of discussion, I am running sbcl-1.0.35 on 32 bit
linux, although I see the same issues in 64 bit linux, and 32 bit x86 OS X.)

Brett van de Sande