(PECL libevent >= 0.0.1)
event_buffer_watermark_set — Set the watermarks for read and write events
$bevent
   , int $events
   , int $lowmark
   , int $highmark
   )
   Sets the watermarks for read and write events. Libevent does not invoke
   read callback unless there is at least lowmark bytes
   in the input buffer; if the read buffer is beyond the highmark, 
   reading is stopped. On output, the write callback is invoked whenever the
   buffered data falls below the lowmark. 
  
beventValid buffered event resource.
events
       Any combination of EV_READ and
       EV_WRITE.
      
lowmarkLow watermark.
highmarkHigh watermark.