Tcl Source Code

When a fileevent is set up to include the data read from the channel, the performance is dramatically reduced due to shimmering from list to string in Tcl_FileEventObjCmd().

Script: set up a pipeline, and on each read in the fileevent, reset the fileevent script to include the new data. I have intentionally limited the read to 65536 bytes at one time in order to show the problem.

proc read_channel {fd data} {
    append data [read $fd 65536]
    # puts stderr "[string length $data]"
    if {[eof $fd]} {
	set now [clock milliseconds]
	puts stderr "read [string length $data] bytes in [expr {$now-$::start}]ms"
	catch {close $fd}
	set ::forever 1
	return
    }
    fileevent $fd readable [list read_channel $fd $data]
}
set fd [open "|dd if=/dev/zero bs=65536 count=200" rb]
fconfigure $fd -blocking 0
set start [clock milliseconds]
fileevent $fd readable [list read_channel $fd ""]
vwait forever

Tcl_FileEventObjCmd() is checking whether to remove the fileevent by issuing a TclGetString() on the fileevent script and checking whether the returned string is '\0'

generic/tclIO.c #9285
    /*
     * If we are supposed to delete a stored script, do so.
     */
    if (*(TclGetString(objv[3])) == '\0') {
	DeleteScriptRecord(interp, chanPtr, mask);
	return TCL_OK;
    }

This causes the fileevent script to shimmer into a string each time the filevent is reset.

Proposed patch: I think it is safe to assume that fileevent scripts should be valid lists, so we could check the list length instead. If I do so, the perfomance is greatly enhanced (20x).

Original tcl9.0.3 read 13107200 bytes in 10245ms

With proposed patch applied: read 13107200 bytes in 529ms