October 2010

If we have a binary tree, how do we determine if a key is in the tree? Or another way, how do we know it was inserted correctly? We need to visit every node.

This is a recursive problem and hard to do in gdb. The convenience variables are very simple and we can not implement a stack with them. Since they are effectively global, we can not even hide them in stack frames. What we can do is use concatenation and a node-id to generate uniqe names.

Note that you have to do all variable name declarations before you do any recursion. That statement will make sense when you violate it and wonder why your output looks strange.

define indentby
        printf "%*c", $arg0+1, 0
end

define walk_btree
        set $node_$arg0 = ($arg1)
        set $fnum_$arg0 = ($arg2)
        set $branch_$arg0 = ($arg3)
        set $space_$arg0 = ($arg4)
        set $leftid_$arg0 = $arg0 + 1
        set $rightid_$arg0 = $arg0 + 2

        indentby $space_$arg0
        printf "%d %u (btree *)%p\n", $branch_$arg0, $node_$arg0->e_type[$fnum_$arg0].key, $node_$arg0

        set $space_$arg0 = $space_$arg0 + 1
        if $node_$arg0->e_type[$fnum_$arg0].left
                walk_btree $leftid_$arg0 $node_$arg0->e_type[$fnum_$arg0].left $fnum_$arg0 0 $space_$arg0
        end
        if $node_$arg0->e_type[$fnum_$arg0].right
                walk_btree $rightid_$arg0 $node_$arg0->e_type[$fnum_$arg0].right $fnum_$arg0 1 $space_$arg0
        end
end
document walk_btree
        NODE-ID NODE FNUM BRANCH INDENTBY
        Recursively walk the binary tree printing out the key for the fnum'ed entry
end

define start_walk_btree
        walk_btree 0 $arg0 $arg1 3 1
end
document start_walk_btree
        NODE FNUM
        Recursively walk the binary tree printing out the key for the fnum'ed entry
end

Contrast that against a very simple algorithm to find if the key is in the tree (with the assumption the tree is valid).

Recursion is not needed as we lop of a branch with each iteration.

define find_btree
        set $node = $arg0
        set $fnum = $arg1
        set $key = $arg2
        set $process = 1

        while $node && $process
                if $key == $node->e_type[$fnum].key
                        set $process = 0
                else

                        if $key < $node->e_type[$fnum].key
                                $node = $node->e_type[$fnum].left
                        else
                                $node = $node->e_type[$fnum].right
                        end
                end
        end

        if $node
                printf "Found match on (btree *)%p\n", $node
        else
                printf "No matching node in the binary tree\n"
        end
end

pktt on a filer can capture for all network interfaces on a head. In order to quickly determine which have relevant data, we can use xargs:

KinMage:csiqa-6080-15 thomas$ ls -1 *021* | xargs -I {} -t tshark -R nfs -r {}
tshark -R nfs -r e0a_20101021_090354.trc
tshark -R nfs -r e0b_20101021_090354.trc
tshark -R nfs -r e0c_20101021_090354.trc
tshark -R nfs -r e0d_20101021_090354.trc
tshark -R nfs -r e0e_20101021_090354.trc
122   0.265844 10.227.74.89 -> 10.227.67.203 NFS V4 COMPOUND Call <EMPTY> PUTFH;CLOSE;GETATTR
123   0.267673 10.227.67.203 -> 10.227.74.89 NFS V4 COMPOUND Reply (Call In 122) <EMPTY> PUTFH;CLOSE
tshark -R nfs -r e0f_20101021_090354.trc
tshark -R nfs -r e4a_20101021_090354.trc
tshark -R nfs -r lo_20101021_090354.trc
tshark -R nfs -r losk_20101021_090354.trc
KinMage:csiqa-6080-15 thomas$ cd ../csiqa-6080-16/
KinMage:csiqa-6080-16 thomas$ ls -1 *021* | xargs -I {} -t tshark -R nfs -r {}
tshark -R nfs -r e0a_20101021_090347.trc
tshark -R nfs -r e0b_20101021_090347.trc
tshark -R nfs -r e0c_20101021_090347.trc
tshark -R nfs -r e0d_20101021_090347.trc
tshark -R nfs -r e4a_20101021_090347.trc
tshark -R nfs -r lo_20101021_090347.trc
tshark -R nfs -r losk_20101021_090347.trc

So we see the first head saw NFS traffic on e0e, but the second saw none.

[thomas@godwit ~]> tshark -R nfs -r e4a_20101020_223437.trc
9   1.020806 10.225.212.66 -> 10.225.212.116 NFS V4 NULL Call
10   1.020972 10.225.212.116 -> 10.225.212.66 NFS V4 NULL Reply (Call In 9)