---
title: Prompt Caching
---

Prompt Caching or Automatic Prefix Caching (APC in short) caches the KV cache of existing queries, so that a new query can directly reuse the KV cache if it shares the same prefix with one of the existing queries, allowing the new query to skip the computation of the shared part.

:::tip
Technical details of the implementation are in the next page!
:::

## Using APC in Aphrodite
Simply set `enable_prefix_caching=True` in the Engine, or pass `--enable-prefix-caching` to the CLI.

Here's an example:

```py
import time
from aphrodite import LLM, SamplingParams


# A prompt containing a large markdown table. The table is randomly generated by GPT-4.
LONG_PROMPT = "You are a helpful assistant in recognizes the content of tables in markdown format. Here is a table as follows.\n# Table\n" + """
| ID  | Name          | Age | Occupation    | Country       | Email                  | Phone Number   | Address                       |
|-----|---------------|-----|---------------|---------------|------------------------|----------------|------------------------------|
| 1   | John Doe      | 29  | Engineer      | USA           | john.doe@example.com   | 555-1234       | 123 Elm St, Springfield, IL  |
| 2   | Jane Smith    | 34  | Doctor        | Canada        | jane.smith@example.com | 555-5678       | 456 Oak St, Toronto, ON      |
| 3   | Alice Johnson | 27  | Teacher       | UK            | alice.j@example.com    | 555-8765       | 789 Pine St, London, UK      |
| 4   | Bob Brown     | 45  | Artist        | Australia     | bob.b@example.com      | 555-4321       | 321 Maple St, Sydney, NSW    |
| 5   | Carol White   | 31  | Scientist     | New Zealand   | carol.w@example.com    | 555-6789       | 654 Birch St, Wellington, NZ |
| 6   | Dave Green    | 28  | Lawyer        | Ireland       | dave.g@example.com     | 555-3456       | 987 Cedar St, Dublin, IE     |
| 7   | Emma Black    | 40  | Musician      | USA           | emma.b@example.com     | 555-1111       | 246 Ash St, New York, NY     |
| 8   | Frank Blue    | 37  | Chef          | Canada        | frank.b@example.com    | 555-2222       | 135 Spruce St, Vancouver, BC |
| 9   | Grace Yellow  | 50  | Engineer      | UK            | grace.y@example.com    | 555-3333       | 864 Fir St, Manchester, UK   |
| 10  | Henry Violet  | 32  | Artist        | Australia     | henry.v@example.com    | 555-4444       | 753 Willow St, Melbourne, VIC|
| 11  | Irene Orange  | 26  | Scientist     | New Zealand   | irene.o@example.com    | 555-5555       | 912 Poplar St, Auckland, NZ  |
| 12  | Jack Indigo   | 38  | Teacher       | Ireland       | jack.i@example.com     | 555-6666       | 159 Elm St, Cork, IE         |
| 13  | Karen Red     | 41  | Lawyer        | USA           | karen.r@example.com    | 555-7777       | 357 Cedar St, Boston, MA     |
| 14  | Leo Brown     | 30  | Chef          | Canada        | leo.b@example.com      | 555-8888       | 246 Oak St, Calgary, AB      |
| 15  | Mia Green     | 33  | Musician      | UK            | mia.g@example.com      | 555-9999       | 975 Pine St, Edinburgh, UK   |
| 16  | Noah Yellow   | 29  | Doctor        | Australia     | noah.y@example.com     | 555-0000       | 864 Birch St, Brisbane, QLD  |
| 17  | Olivia Blue   | 35  | Engineer      | New Zealand   | olivia.b@example.com   | 555-1212       | 753 Maple St, Hamilton, NZ   |
| 18  | Peter Black   | 42  | Artist        | Ireland       | peter.b@example.com    | 555-3434       | 912 Fir St, Limerick, IE     |
| 19  | Quinn White   | 28  | Scientist     | USA           | quinn.w@example.com    | 555-5656       | 159 Willow St, Seattle, WA   |
| 20  | Rachel Red    | 31  | Teacher       | Canada        | rachel.r@example.com   | 555-7878       | 357 Poplar St, Ottawa, ON    |
| 21  | Steve Green   | 44  | Lawyer        | UK            | steve.g@example.com    | 555-9090       | 753 Elm St, Birmingham, UK   |
| 22  | Tina Blue     | 36  | Musician      | Australia     | tina.b@example.com     | 555-1213       | 864 Cedar St, Perth, WA      |
| 23  | Umar Black    | 39  | Chef          | New Zealand   | umar.b@example.com     | 555-3435       | 975 Spruce St, Christchurch, NZ|
| 24  | Victor Yellow | 43  | Engineer      | Ireland       | victor.y@example.com   | 555-5657       | 246 Willow St, Galway, IE    |
| 25  | Wendy Orange  | 27  | Artist        | USA           | wendy.o@example.com    | 555-7879       | 135 Elm St, Denver, CO       |
| 26  | Xavier Green  | 34  | Scientist     | Canada        | xavier.g@example.com   | 555-9091       | 357 Oak St, Montreal, QC     |
| 27  | Yara Red      | 41  | Teacher       | UK            | yara.r@example.com     | 555-1214       | 975 Pine St, Leeds, UK       |
| 28  | Zack Blue     | 30  | Lawyer        | Australia     | zack.b@example.com     | 555-3436       | 135 Birch St, Adelaide, SA   |
| 29  | Amy White     | 33  | Musician      | New Zealand   | amy.w@example.com      | 555-5658       | 159 Maple St, Wellington, NZ |
| 30  | Ben Black     | 38  | Chef          | Ireland       | ben.b@example.com      | 555-7870       | 246 Fir St, Waterford, IE    |
"""


def get_generation_time(llm, sampling_params, prompts):
    # time the generation
    start_time = time.time()
    output = llm.generate(prompts, sampling_params=sampling_params)
    end_time = time.time()
    # print the output and generation time
    print(f"Output: {output[0].outputs[0].text}")
    print(f"Generation time: {end_time - start_time} seconds.")


# set enable_prefix_caching=True to enable APC
llm = LLM(
    model='lmsys/longchat-13b-16k',
    enable_prefix_caching=True
)

sampling_params = SamplingParams(temperature=0, max_tokens=100)

# Querying the age of John Doe
get_generation_time(
    llm,
    sampling_params,
    LONG_PROMPT + "Question: what is the age of John Doe? Your answer: The age of John Doe is ",
)

# Querying the age of Zack Blue
# This query will be faster since Aphrodite avoids computing the KV cache of LONG_PROMPT again.
get_generation_time(
    llm,
    sampling_params,
    LONG_PROMPT + "Question: what is the age of Zack Blue? Your answer: The age of Zack Blue is ",
)
```