Error load test api login using FastAPI and locust

Question

I have written the simple code about login user, generate access token by fastapi:

import fastapi as _fastapi
import fastapi.security as _security

import sqlalchemy.orm as _orm

import services as _services
import schemas as _schemas

app = _fastapi.FastAPI()


@app.post("/api/users")
async def create_user(user: _schemas.UserCreate, db: _orm.Session = _fastapi.Depends(_services.get_db)):
    db_user = await _services.get_user_by_email(email=user.email, db=db)
    if db_user:
        raise _fastapi.HTTPException(
            status_code=400,
            detail="User with that email already exists")

    user = await _services.create_user(user=user, db=db)

    return await _services.create_token(user=user)


@app.post("/api/token")
async def generate_token(
                        form_data: _security.OAuth2PasswordRequestForm = _fastapi.Depends(),
                        db: _orm.Session = _fastapi.Depends(_services.get_db)):
    user = await _services.authenticate_user(
        email=form_data.username, password=form_data.password, db=db)

    if not user:
        raise _fastapi.HTTPException(
            status_code=401, detail="Invalid Credentials")

    return _services.create_token(user=user)

database config:

import sqlalchemy as _sql
import sqlalchemy.ext.declarative as _declarative
import sqlalchemy.orm as _orm

DATABASE_URL = 'postgresql://localhost:5432/test1'

engine = _sql.create_engine(
    DATABASE_URL, pool_size=15, max_overflow=5)

SessionLocal = _orm.sessionmaker(
    autocommit=False, autoflush=False, bind=engine)

Base = _declarative.declarative_base()

I use uvicorn and locust to load test but it has very small rps and limit number request. Locust have error:

 POST /api/token: RemoteDisconnected('Remote end closed connection without response')                
 POST /api/token: ConnectionResetError(54, 'Connection reset by peer') 

I try to change connection pool and upgrade request[sercurity] but inoperative.