InstanceContextMode of WCF -

i have web service create wcf, , response client's request, such adding or updating data.

let's call main part of service context, because method(supposed method a) spend long time handle, don't want method hold server, set instancecontextmode enum percall, , expect each time call a, create new instance serve request, server continue server other one's usual.

but problem is, still use main thread(ui thread) serve each time, when call a, server stop response long time, till a done.

did miss anything? i'm novice of wcf, please let me know if had mistake.