posmaster failed under high load

Today I run http_load to do some benchmark of my Web-DB application
and found that under high load (it was about 18 postgres running ) postsmaster failed.
Restarting postmaster produces error:

IpcMemoryCreate: shmget failed (Identifier removed) key=5432010, size=24588, permission=700
IpcMemoryIdGet: shmget failed (Identifier removed) key=5432010, size=24588, permission=0
IpcMemoryAttach: shmat failed (Invalid argument) id=-2
FATAL 1:  AttachSLockMemory: could not attach segment

I checked shared memory:
23:27[zeus]:~>ipcs -a

------ Shared Memory Segments --------
shmid     owner     perms     bytes     nattch    status      
10496     postgres  700       24588     5         dest        
10497     postgres  600       8852184   5         dest        
10498     postgres  600       96804     5         dest        

------ Semaphore Arrays --------
semid     owner     perms     nsems     status      

------ Message Queues --------
msqid     owner     perms     used-bytes  messages    

Does this error means I need to increase shared memory ?
My setup: Linux 2.0.36, Dual PPRO, 256 Mb RAM
Postgres 6.4.2
Regards,
    Oleg


_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83