to speed up performance of set-with-expire

[kaitian521]

I tested performance for expire-enabled and expire-disabled

    int size = 20000;
    auto mmkv0 = MMKV::mmkvWithID("testSpeedForExpire0", MMKV_SINGLE_PROCESS, nullptr, nullptr, size);
    mmkv0->enableAutoKeyExpire(24 * 60 * 60 * 1000);
    auto mmkv1 = MMKV::mmkvWithID("testSpeedForExpire1", MMKV_SINGLE_PROCESS, nullptr, nullptr, size);

    mmkv0->clearAll();
    mmkv1->clearAll();

    string key = "key1";
    int times = 500000;
    string bigValue(20000, 0);
    uint64_t start, end;

    start = getTimeInMs();
    for (int i = 0; i < times; i++) {
        mmkv0->set(bigValue, key);
        mmkv0->removeValueForKey(key);
    }
    end = getTimeInMs();
    printf("%llu\n", end - start);

    start = getTimeInMs();
    for (int i = 0; i < times; i++) {
        mmkv1->set(bigValue, key);
        mmkv1->removeValueForKey(key);
    }
    end = getTimeInMs();
    printf("%llu\n", end - start);

Two tests have no sync, no expand， only overrideWrite

expire-enabled: 2253ms
expire-disabled:1748ms
This means expire-enabled is 28% slower than expire-disabled.

bool MMKV::set(const string &value, MMKVKey_t key, uint32_t expireDuration) {
    if (isKeyEmpty(key)) {
        return false;
    }
    if (likely(!m_enableKeyExpire)) {
        assert(expireDuration == ExpireNever && "setting expire duration without calling enableAutoKeyExpire() first");
        return setDataForKey(MMBuffer((void *) value.data(), value.length(), MMBufferNoCopy), key, true);
    } else {
        MMBuffer data((void *) value.data(), value.length(), MMBufferNoCopy);
        if (data.length() > 0) {
            auto tmp = MMBuffer(pbMMBufferSize(data) + Fixed32Size);
            CodedOutputData output(tmp.getPtr(), tmp.length());
            output.writeData(data);
            auto time = (expireDuration != ExpireNever) ? getCurrentTimeInSecond() + expireDuration : ExpireNever;
            output.writeRawLittleEndian32(UInt32ToInt32(time));
            data = std::move(tmp);
        }
        return setDataForKey(std::move(data), key);
    }
}

Maybe it exist some improvement here, by forbidding to allocate new MMBuffer.
For example, we make isDataHolder is true, pass the origin value to setDataForKey, when real write to file, carefully handle the 4 bytes const expire time.

    // My-adding: carefully calculated total size
    if (isDataHolder) {
       // My-adding: let valueLength add 4 if expire-enabled
        auto valueLength = getValueLength(data, isDataHolder);
        output->writeRawVarint32((int32_t) valueLength);
    }
    output->writeData(data); // note: write size of data
       // My-adding: write 4 byte expire time

ONLY my ideas.

[lingol]

Sure but it requires much more work and the performance gain is notable only with large values.