Setting deadlock_priority does not make LOW priority session the victim

Question

One of our jobs was deadlocking with user routines, so we put the following code in the job step, just before the procedure is called:

DECLARE @deadlock_var NCHAR(3);  
SET @deadlock_var = N'LOW';  
SET DEADLOCK_PRIORITY @deadlock_var;
*--Call procedure
exec Client_myDeliveries_I_S*

However, the procedure is still in the stack that is processed. Does the deadlock priority not get inherited through the chain of sub procedures? I can confirm that at no point does the deadlock victim session set its deadlock_priority.

The complete deadlock XML is below, with some deletions for privacy:

<deadlock>
 <victim-list>
  <victimProcess id="process28fc21868" />
 </victim-list>
 <process-list>
  <process id="process28fc21868" taskpriority="0" logused="2504" waitresource="KEY: 5:72057594562412544 (40fd182c0dd9)" waittime="5008" ownerId="299034576" transactionname="user_transaction" lasttranstarted="2018-02-01T12:22:55.580" XDES="0x140b2cc70" lockMode="X" schedulerid="1" kpid="3600" status="suspended" spid="87" sbid="0" ecid="0" priority="0" trancount="2" lastbatchstarted="2018-02-01T12:22:55.580" lastbatchcompleted="2018-02-01T12:22:55.580" lastattention="2018-02-01T12:22:52.480" clientapp="EUROSTOP e-i Service" hostname="SRVAZBRWSQL01" hostpid="1328" loginname="sa" isolationlevel="read committed (2)" xactid="299034576" currentdb="5" lockTimeout="4294967295" clientoption1="673316896" clientoption2="128056">
   <executionStack>
    <frame procname="ALL.dbo.trgi_u_Constants" line="705" stmtstart="37030" stmtend="37270" sqlhandle="0x03000500651fce5aad8fd3002aa5000000000000000000000000000000000000000000000000000000000000">
update myconstants
                set value = (select last_c_no from inserted)
                where batch = 'last_c_no'    </frame>
    <frame procname="IF_TEST.myschema.SendTestData_Customers_SToALL_Customers" line="87" stmtstart="11420" stmtend="11650" sqlhandle="0x030007007f19ab663ac3e5005ca7000001000000000000000000000000000000000000000000000000000000">
update myschema.ALL_Constants
            set     last_c_no = @nLastCNumber + @NumberOfInsertedCs    </frame>
    <frame procname="IF_TEST.myschema.SendTestData_Customers" line="22" stmtstart="962" stmtend="1196" sqlhandle="0x030007009c954b7757b4d00054a7000001000000000000000000000000000000000000000000000000000000">
exec myschema.SendTestData_Customers_SToALL_Customers @MessageCode, @RejectAllOnValidationError    </frame>
    <frame procname="IF_TEST.myschema.SendSubmittedFData" line="13" stmtstart="876" stmtend="1114" sqlhandle="0x030007009b4fb66e2db4d00054a7000001000000000000000000000000000000000000000000000000000000">
exec [myschema].[SendTestData_Customers] @MessageCode, @RejectAllOnValidationError

    -- Customer Orders    </frame>
    <frame procname="adhoc" line="1" stmtstart="104" sqlhandle="0x010007008547740e50dc4dd90700000000000000000000000000000000000000000000000000000000000000">
Exec myschema.SendSubmittedFData @0, @1, @2, @3    </frame>
    <frame procname="unknown" line="1" sqlhandle="0x0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000">
unknown    </frame>
   </executionStack>
   <inputbuf>
(@0 nvarchar(4000),@1 int,@2 nvarchar(4000),@3 int);Exec myschema.SendSubmittedFData @0, @1, @2, @3   </inputbuf>
  </process>
  <process id="process2b6e94cf8" taskpriority="-6" logused="43652" waitresource="KEY: 5:72057594562412544 (d08358b1108f)" waittime="5009" ownerId="299033786" transactionname="user_transaction" lasttranstarted="2018-02-01T12:22:53.810" XDES="0x28262f130" lockMode="X" schedulerid="2" kpid="13408" status="suspended" spid="122" sbid="0" ecid="0" priority="-5" trancount="3" lastbatchstarted="2018-02-01T12:15:00.580" lastbatchcompleted="2018-02-01T12:15:00.580" lastattention="1900-01-01T00:00:00.580" clientapp="SQLAgent - TSQL JobStep (Job 0x24121E41ABD80643985B522FE6C248A7 : Step 1)" hostname="SRVAZBRWSQL01" hostpid="2252" loginname="SRVAZBRWSQL01\ALLSYSTEM" isolationlevel="read committed (2)" xactid="299033786" currentdb="5" lockTimeout="4294967295" clientoption1="673316896" clientoption2="128056">
   <executionStack>
    <frame procname="ALL.dbo.trgi_u_Constants" line="726" stmtstart="38142" stmtend="38386" sqlhandle="0x03000500651fce5aad8fd3002aa5000000000000000000000000000000000000000000000000000000000000">
update myconstants
                set value = (select last_gs_rec_no from inserted)
                where batch = 'last_gs_rec_no'    </frame>
    <frame procname="ALL.dbo.trgi_i_i_I_deliveries" line="1005" stmtstart="69046" stmtend="69220" sqlhandle="0x030005006cd104389a850d00a1a6000000000000000000000000000000000000000000000000000000000000">
update dbo.Constants set last_gs_rec_no = last_gs_rec_no + @nmyDeliveriesCreatedCount    </frame>
    <frame procname="C_HELP.dbo.Client_myDeliveries_I_Std" line="86" stmtstart="7306" stmtend="8324" sqlhandle="0x030006000d09a1438efcba0074a8000001000000000000000000000000000000000000000000000000000000">
insert  into [ALL].dbo.i_I_deliveries
                --[columns]
                select  --[columns]
                from    #Client_deliveries_PO_stg stg    </frame>
    <frame procname="C_HELP.dbo.Client_myDeliveries_I_S" line="446" stmtstart="32832" stmtend="33032" sqlhandle="0x0300060028323b37f7a70101eca7000001000000000000000000000000000000000000000000000000000000">
exec  dbo.Client_myDeliveries_I_Std @week_selector, @username, @factory_bin_location, @Parameter    </frame>
    <frame procname="adhoc" line="6" stmtstart="216" sqlhandle="0x02000000e49e3111676b7e3aec714d06946692f70e3a8a880000000000000000000000000000000000000000">
exec Client_myDeliveries_I_S    </frame>
   </executionStack>
   <inputbuf>
DECLARE @deadlock_var NCHAR(3);  
SET @deadlock_var = N'LOW';  

SET DEADLOCK_PRIORITY @deadlock_var;

exec Client_myDeliveries_I_S   </inputbuf>
  </process>
 </process-list>
 <resource-list>
  <keylock hobtid="72057594562412544" dbid="5" objectname="ALL.dbo.myconstants" indexname="pk_myconstants" id="lockd5ad7b00" mode="RangeS-U" associatedObjectId="72057594562412544">
   <owner-list>
    <owner id="process2b6e94cf8" mode="RangeS-S" />
   </owner-list>
   <waiter-list>
    <waiter id="process28fc21868" mode="X" requestType="convert" />
   </waiter-list>
  </keylock>
  <keylock hobtid="72057594562412544" dbid="5" objectname="ALL.dbo.myconstants" indexname="pk_myconstants" id="lock9466df00" mode="RangeS-U" associatedObjectId="72057594562412544">
   <owner-list>
    <owner id="process28fc21868" mode="RangeS-S" />
   </owner-list>
   <waiter-list>
    <waiter id="process2b6e94cf8" mode="X" requestType="convert" />
   </waiter-list>
  </keylock>
 </resource-list>
</deadlock>

Additional Version: Microsoft SQL Server 2012 (SP3-CU2) (KB3137746) - 11.0.6523.0 (X64) Mar 2 2016 21:29:16 Copyright (c) Microsoft Corporation Standard Edition (64-bit) on Windows NT 6.3 (Build 9600: ) (Hypervisor)

Answer 1

From everyone's input, it seems that deadlock priority can be ignored by SQL Server under circumstances that have not been made clear by Microsoft (that we know of); the algorithms used to determine the victim are not understood or documented. If anyone can find solid information on this topic, please share.