VPR Placer runtime issue when all design clusters have fixed locations about vtr-verilog-to-routing

[soheilshahrouz]

Thank you for opening this issue.

It seems that some move generators call pick_from_block() function to select a block randomly. This function has a while loop that tries to find a movable block. Since all blocks in your run are fixed, this loop's exit condition is not met until all possible options (all clustered blocks) are exhaused.

vtr-verilog-to-routing/vpr/src/place/move_utils.cpp

Lines 595 to 626 in 451fb4d

	ClusterBlockId pick_from_block() {
	/* Some blocks may be fixed, and should never be moved from their *
	* initial positions. If we randomly selected such a block try *
	* another random block. *
	* *
	* We need to track the blocks we have tried to avoid an infinite *
	* loop if all blocks are fixed. */
	auto& cluster_ctx = g_vpr_ctx.clustering();
	auto& place_ctx = g_vpr_ctx.mutable_placement();
	std::unordered_set<ClusterBlockId> tried_from_blocks;
	//Keep selecting random blocks as long as there are any untried blocks
	//Can get slow if there are many blocks but only a few (or none) can move
	while (tried_from_blocks.size() < cluster_ctx.clb_nlist.blocks().size()) {
	//Pick a block at random
	ClusterBlockId b_from = ClusterBlockId(vtr::irand((int)cluster_ctx.clb_nlist.blocks().size() - 1));
	//Record it as tried
	tried_from_blocks.insert(b_from);
	if (place_ctx.block_locs[b_from].is_fixed) {
	continue; //Fixed location, try again
	}
	//Found a movable block
	return b_from;
	}
	//No movable blocks found
	return ClusterBlockId::INVALID();
	}

If you want to skip placement altogether, I guess you can pass the placement file using --place_file option.

@vaughnbetz What is you opinion on this? Should I change pick_from_block() function? We can fix this by trying only a few clustered blocks to find a movable one. Alternatively, we can store all movable blocks in a separate container and select an element of this container randomly.

[vaughnbetz]

Thanks @soheilshahrouz . Probably we should put all the movable blocks (and only movable blocks) in a container and select them randomly, so we are always efficient.

[rachelselinar]

I was able to convert a .fix_clusters file to a .place file by including '0' subtile entry for all entries and appending netlist checksum and array information as a header.

By passing the .place file using the --place_file option, router ran successfully (and much faster) and the results are the same.

Thank you

[vaughnbetz]

Great, thanks Rachel. I think without the checksum it will just give a warning, so if that's a pain to maintain I think you can skip it (it's intended to protect people from accidentally using a .place file for the wrong circuit or architecture).

[rachelselinar]

I was able to obtain the netlist checksum and array size information from runs that did not fix all clusters in the .fix_clusters file. In addition, VPR placer errors out if the .place file doesn't have these 3 lines in its header

1. Netlist checksum
2. Array size

Without either (or 3 empty lines as header), placer errors out at the first entry:
_

Error 1:
Type: Placement file
File: ../gaussianblur.place
Line: 3
Message: Invalid line 'step0:grp_step0_fu_168|step0_grp_fu_2167_ACMP_fmul_6:step0_grp_fu_2167_ACMP_fmul_6_U|ACMP_fmul:ACMP_fmul_U|AESL_WP_FMul:ACMP_FMul_U|lpm_mult:Mult0|mult_8at:auto_generated|mac_mult2 166 126 0 0 #0' in placement file header

_

3. Empty line

If only the checksum and array size is provided without an empty line, the placer errors out:

_

Error 1:
Type: Placement
File: ~/vtr-verilog-to-routing/vpr/src/base/read_place.cpp
Line: 297
Message: Block 0 has not been read from the place file.

_

For gaussianblur, the .place file generated by VPR placer contains this header:

Netlist_File: gaussianblur.net Netlist_ID: SHA256:d6e68433c9959dc959dc0758f8f9f026cd79b00d8fbbfce6c211b820e50589dd
Array size: 400 x 296 logic blocks

#block name x y subblk layer block number
#---------- -- -- ------ ----- ------------

[vaughnbetz]

Thanks; we should probably make this a bit more robust (keep parsing and give a warning).