If your buffer pool is very large, or your applications do a lot of updates to data, you should increase the NUM_IO_CLEANERS value.
The values is related to AWP (asyncronous write percentage) which formula is:
AWP= ((Asyncronous pool data page writes + Asyncronous pool index page writes) x 100) / (Buffer pool data writes + Buffer pool index writes).
This formula applies equally well to database, buffer pool and tablespace snapshot data. For good performance it's recommended set AWP 90 or higher. So, increase the number of NUM_IO_CLEANERS by one until either 90 percent of writes are performed asyncronously or the number of NUM_IO_CLEANERS is equal to the number of CPUs on the db2 server. If the latter limit is reached, consider making gradual reductions in CHNGPGS_THRES until AWP is greater than 90 but set CHNGPGS_THRES no lower than 30 procent. (info: db2 magazine vol 7 number 3, 2002, Hayes&Gunning)