mysql - how to simulate slow update sql? -

dba told me there slow sql should optimize,

and there 10 same update sql , longest time 56 seconds.

the business logic of sql that, if user wins award , should decrease 1 stock of award.

i think caused high concurrent, in case exist lock competion. decided repropduce in local environment, let many thread execute update operation simultaneously, cannot reproduce above slow sql, below experiment result:

thred count|100|500|1000|2000 -----|-----|-----|-----|----- took time(ms)|149|490|1064|1996 

even 2000 update sql execute concurrently, need 1996ms execute all. how reprduce these slow sql?

my code

@test public void test_concurrent_update() throws interruptedexception {     int nthreads = 50; //thread count      executorservice pool = executors.newfixedthreadpool(nthreads);     final countdownlatch startlatch = new countdownlatch(nthreads);     final countdownlatch endlatch = new countdownlatch(nthreads);     long starttime = system.currenttimemillis();     (int = 0; < nthreads; i++) {         pool.submit(new runnable() {             @override             public void run() {                 startlatch.countdown(); // let threads concurrently execute update operation                 try { startlatch.await(); } catch (interruptedexception e) { e.printstacktrace(); }                 mapper.updateawardstock(); // reduce stock mybatis                 endlatch.countdown();                 system.out.printf("%s down%n",thread.currentthread().getname());             }         });     }      endlatch.await(); // wait threads finished executing     long endtime = system.currenttimemillis();     system.out.printf("%d threads concurrent update stock took time: %d(ms)%n",nthreads,(endtime-starttime));  }  @update("update award_stock set stock = stock-1 award_id = 1 ,  stock>0 limit 1") void updateawardstock();