sorl-thumbnail does not create cache

I'm using sorl-thumbnail in django. I followed the installation guide. But thumbnail seems not working. It cannot generate cache folder under MEDIA_URL. Also, I checked my mysql database. Table thumbnail_kvstore is always empty.

In settings.py:

INSTALLED_APPS = (
...
'sorl.thumbnail',
)
MEDIA_ROOT = os.path.join(BASE_DIR, 'media/').replace('\\','/')
MEDIA_URL = '/media/'

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'LOCATION': '127.0.0.1:11211',
    }
}

In urls.py:

urlpatterns = [
url(r'^admin/', include(admin.site.urls)),
...
] + static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)

In models.py:

from sorl.thumbnail import ImageField

class Image(models.Model):
    file = models.ImageField(upload_to="images")
    user = models.ForeignKey(User)

In template:

{% thumbnail image "100x100" crop="center" as im %}
    <img src="{{ im.url }}" width="{{ im.width }}" height="{{ im.height }}">
{% endthumbnail %}

It turns out that the im.url generated by thumbnail looks like "/media/cache/6f/9d/6f9dc8d01bf2a23448525d0902a63cd6.jpg". But there isn't a cache folder generated under media/. And thumbnail_kvstore in my database is always empty. Can anyone help me here?

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