Similarities and differences between the wait-die and the wound-wait scheme

Similarity:

Both schemes are fair:

When transactions restart, they keep their timestamps
Eventually, the aborted (younger) transactions will become the oldest transactions in the system

Difference:

The cost of aborting (younger/older) transactions

Facts:

Younger transactions (having just started running) will typically:
Older transactions (having run longer) will typically:
Older transactions (= that started earlier) are more expensive (costly) to be aborted !!!!

Comparing the abort rates of the wait-die and would-wait schemes

Predicted transaction abort rates in wait-die and wound-wait methods:

The number of transactions that will be aborted in the wound-wait method will be

Reason:

A younger transaction starts later than an older transaction:
By the time that the younger transaction T₂ starts, the older transactions will have obtained most of their locks:
A younger (just starting) transaction T₂ will make many lock requests !!
In the wound-wait method, a younger (just starting) transaction T₂ will wait (and not abort) if T₂ requests a lock held by the older transaction T₁:
An older transaction (T₁) that requests a lock can abort a younger transaction (T₂):
However:
Therefore:

Wait-die method will have more aborts

(Similar) reason:

A younger transaction starts later than an older transaction:
By the time that T₂ starts, the older transactions will have obtained most of their locks:
A younger (just starting) transaction T₂ will make many lock requests !!
In the wait-die method, a younger (just starting) transaction T₂ will die (= abort) if T₂ requests a lock held by the older transaction T₁:
Since younger transactions makes many lock requests:

Comparing the amount of waste work in aborted transaction in wait-die and wound-wait

The wound-wait method may have fewer aborts, however; it's likely that:

An aborted (younger) transaction in the wound-wait method has performed more work than an aborted (younger) transaction in the wait-die method !!!

Argument:

Consider a transaction in the wound-wait method that gets aborted:
Notice that the aborted transaction must hold some locks !!!
I.e.: the aborted transaction has performed some read/write operations (= useful work):
The useful work is erased (rolled back) !!!
Consider a transaction in the wait-die method that gets aborted:
Notice that the aborted transaction may not hold any lock at all !!!
I.e.: the aborted transaction has not performed any read/write operation (= no useful work):

Consider the previous examples:

Wait-die locking method:

Notice:

There are 2 transaction aborts (that's 50% of the # transactions !!!)
However:

Wound-wait locking method:

Notice:

There is only 1 transaction aborts (less than in wait-die !!!)
However:

Bottom line:

Wait-die has more roll backs; but the transactions have perform little to no work
Wound-wait has less roll backs; but the transaction that got aborted has done some work !!!