bachelor-thesis/offloading-cacher/cache.hpp

#pragma once

#include <iostream>

#include <unordered_map>
#include <shared_mutex>
#include <mutex>
#include <memory>

#include <sched.h>
#include <numa.h>
#include <numaif.h>

#include <dml/dml.hpp>

#include "cache-data.hpp"

namespace dsacache {
    // cache class will handle access to data through the cache
    // by managing the cache through work submission, it sticks
    // to user-defined caching and copy policies, is thread
    // safe after initialization and returns copies of
    // cache data class to the user

    class Cache {
    public:
        // cache policy is defined as a type here to allow flexible usage of the cacher
        // given a numa destination node (where the data will be needed), the numa source
        // node (current location of the data) and the data size, this function should
        // return optimal cache placement
        // dst node and returned value can differ if the system, for example, has HBM
        // attached accessible directly to node n under a different node id m
        typedef int (CachePolicy)(const int numa_dst_node, const int numa_src_node, const size_t data_size);

        // copy policy specifies the copy-executing nodes for a given task
        // which allows flexibility in assignment for optimizing raw throughput
        // or choosing a conservative usage policy
        typedef std::vector<int> (CopyPolicy)(const int numa_dst_node, const int numa_src_node);

    private:
        // mutex for accessing the cache state map

        std::shared_mutex cache_mutex_;

        // map from [dst-numa-node,map2]
        // map2 from [data-ptr,cache-structure]

        std::unordered_map<uint8_t, std::unordered_map<uint8_t*, CacheData>> cache_state_;

        CachePolicy* cache_policy_function_ = nullptr;
        CopyPolicy* copy_policy_function_ = nullptr;

        // function used to submit a copy task on a specific node to the dml
        // engine on that node - will change the current threads node assignment
        // to achieve this so take care to restore this
        dml::handler<dml::mem_copy_operation, std::allocator<uint8_t>> ExecuteCopy(
                const uint8_t* src, uint8_t* dst, const size_t size, const int node
        ) const;

        // allocates the required memory on the destination node
        // and then submits task to the dml library for processing
        // and attaches the handlers to the cache data structure
        void SubmitTask(CacheData* task, const int dst_node, const int src_node);

        // querries the policy functions for the given data and size
        // to obtain destination cache node, also returns the datas
        // source node for further usage
        // output may depend on the calling threads node assignment
        // as this is set as the "optimal placement" node
        void GetCacheNode(uint8_t* src, const size_t size, int* OUT_DST_NODE, int* OUT_SRC_NODE) const;

        // checks whether the cache contains an entry for
        // the given data in the given memory node and
        // returns it, otherwise returns nullptr
        std::unique_ptr<CacheData> GetFromCache(uint8_t* src, const size_t size, const int dst_node);

    public:
        // initializes the cache with the two policy functions
        // only after this is it safe to use in a threaded environment
        void Init(CachePolicy* cache_policy_function, CopyPolicy* copy_policy_function);

        // function to perform data access through the cache
        std::unique_ptr<CacheData> Access(uint8_t* data, const size_t size);

        // flushes the cache of inactive entries
        // if node is -1 then the whole cache is
        // checked and otherwise the specified
        // node - no checks on node validity
        void Flush(const int node = -1);
    };
}

inline void dsacache::Cache::Init(CachePolicy* cache_policy_function, CopyPolicy* copy_policy_function) {
    cache_policy_function_ = cache_policy_function;
    copy_policy_function_ = copy_policy_function;

    // initialize numa library

    numa_available();

    // obtain all available nodes
    // and those we may allocate
    // memory on

    const int nodes_max = numa_num_configured_nodes();
    const bitmask* valid_nodes = numa_get_mems_allowed();

    // prepare the cache state with entries
    // for all given nodes

    for (int node = 0; node < nodes_max; node++) {
        if (numa_bitmask_isbitset(valid_nodes, node)) {
            cache_state_.insert({node,{}});
        }
    }

    std::cout << "[-] Cache Initialized" << std::endl;
}

inline std::unique_ptr<dsacache::CacheData> dsacache::Cache::Access(uint8_t* data, const size_t size) {
    // get destination numa node for the cache

    int dst_node = -1;
    int src_node = -1;

    GetCacheNode(data, size, &dst_node, &src_node);

    // TODO: at this point it could be beneficial to check whether
    // TODO: the given destination node is present as an entry
    // TODO: in the cache state to see if it is valid

    // check whether the data is already cached

    std::unique_ptr<CacheData> task = GetFromCache(data, size, dst_node);

    if (task != nullptr) {
        return std::move(task);
    }

    // at this point the requested data is not present in cache
    // and we create a caching task for it

    task = std::make_unique<CacheData>(data, size);

    {
        std::unique_lock<std::shared_mutex> lock(cache_mutex_);

        const auto state = cache_state_[dst_node].emplace(task->src_, *task);

        // if state.second is false then no insertion took place
        // which means that concurrently whith this thread
        // some other thread must have accessed the same
        // resource in which case we return the other
        // threads data cache structure

        if (!state.second) {
            std::cout << "[!] Found another cache instance for 0x" << std::hex << (uint64_t)task->src_ << std::dec << std::endl;
            return std::move(std::make_unique<CacheData>(state.first->second));
        }
    }

    SubmitTask(task.get(), dst_node, src_node);

    return std::move(task);
}

inline void dsacache::Cache::SubmitTask(CacheData* task, const int dst_node, const int src_node) {
    std::cout << "[+] Allocating " << task->size_ << "B on node " << dst_node << " for " << std::hex << (uint64_t)task->src_ << std::dec << std::endl;

    // allocate data on this node and flush the unused parts of the
    // cache if the operation fails and retry once
    // TODO: smarter flush strategy could keep some stuff cached

    uint8_t* dst = reinterpret_cast<uint8_t*>(numa_alloc_onnode(task->size_, dst_node));

    if (dst == nullptr) {
        std::cout << "[!] First allocation try failed for " << task->size_ << "B on node " << dst_node << std::endl;

        // allocation on dst_node failed so we flush the cache for this
        // node hoping to free enough currently unused entries to make
        // the second allocation attempt successful

        Flush(dst_node);

        dst = reinterpret_cast<uint8_t*>(numa_alloc_onnode(task->size_, dst_node));

        if (dst == nullptr) {
            std::cerr << "[x] Second allocation try failed for " << task->size_ << "B on node " << dst_node << std::endl;
            return;
        }
    }

    task->incomplete_cache_ = dst;

    // querry copy policy function for the nodes to use for the copy

    const std::vector<int> executing_nodes = copy_policy_function_(dst_node, src_node);
    const size_t task_count = executing_nodes.size();

    // each task will copy one fair part of the total size
    // and in case the total size is not a factor of the
    // given task count the last node must copy the remainder

    const size_t size = task->size_ / task_count;
    const size_t last_size = size + task->size_ % task_count;

    std::cout << "[-] Splitting Copy into " << task_count << " tasks of " << size << "B 0x" << std::hex << (uint64_t)task->src_ << std::dec << std::endl;

    // save the current numa node mask to restore later
    // as executing the copy task will place this thread
    // on a different node

    bitmask* nodemask = numa_get_run_node_mask();

    for (uint32_t i = 0; i < task_count; i++) {
        const size_t local_size = i + 1 == task_count ? size : last_size;
        const size_t local_offset = i * size;
        const uint8_t* local_src = task->src_ + local_offset;
        uint8_t* local_dst = dst + local_offset;

        task->handlers_->emplace_back(ExecuteCopy(local_src, local_dst, local_size, executing_nodes[i]));
    }

    // restore the previous nodemask

    numa_run_on_node_mask(nodemask);
    numa_free_nodemask(nodemask);
}

inline dml::handler<dml::mem_copy_operation, std::allocator<uint8_t>> dsacache::Cache::ExecuteCopy(
        const uint8_t* src, uint8_t* dst, const size_t size, const int node
) const {
    dml::const_data_view srcv = dml::make_view(src, size);
    dml::data_view dstv = dml::make_view(dst, size);

    numa_run_on_node(node);

    return dml::submit<dml::automatic>(dml::mem_copy.block_on_fault(), srcv, dstv);
}

void dsacache::Cache::GetCacheNode(uint8_t* src, const size_t size, int* OUT_DST_NODE, int* OUT_SRC_NODE) const {
    // obtain numa node of current thread to determine where the data is needed

    const int current_cpu = sched_getcpu();
    const int current_node = numa_node_of_cpu(current_cpu);

    // obtain node that the given data pointer is allocated on

    *OUT_SRC_NODE = -1;
    get_mempolicy(OUT_SRC_NODE, NULL, 0, (void*)src, MPOL_F_NODE | MPOL_F_ADDR);

    // querry cache policy function for the destination numa node

    *OUT_DST_NODE = cache_policy_function_(current_node, *OUT_SRC_NODE, size);
}

inline void dsacache::Cache::Flush(const int node) {
    std::cout << "[-] Flushing Cache for " << (node == -1 ? "all nodes" : "node " + std::to_string(node)) << std::endl;

    // this lambda is used because below we have two code paths that
    // flush nodes, either one single or all successively

    const auto FlushNode = [](std::unordered_map<uint8_t*,CacheData>& map) {
        // begin at the front of the map

        auto it = map.begin();

        // loop until we reach the end of the map

        while (it != map.end()) {
            // if the iterator points to an inactive element
            // then we may erase it

            if (it->second.Active() == false) {
                // erase the iterator from the map

                map.erase(it);

                // as the erasure invalidated out iterator
                // we must start at the beginning again

                it = map.begin();
            }
            else {
                // if element is active just move over to the next one

                it++;
            }
        }
    };

    {
        // we require exclusive lock as we modify the cache state

        std::unique_lock<std::shared_mutex> lock(cache_mutex_);

        // node == -1 means that cache on all nodes should be flushed

        if (node == -1) {
            for (auto& nc : cache_state_) {
                FlushNode(nc.second);
            }
        }
        else {
            FlushNode(cache_state_[node]);
        }
    }
}

std::unique_ptr<dsacache::CacheData> dsacache::Cache::GetFromCache(uint8_t* src, const size_t size, const int dst_node) {
    // the best situation is if this data is already cached
    // which we check in an unnamed block in which the cache
    // is locked for reading to prevent another thread
    // from marking the element we may find as unused and
    // clearing it

    // lock the cache state in shared-mode because we read

    std::shared_lock<std::shared_mutex> lock(cache_mutex_);

    // search for the data in our cache state structure at the given node

    const auto search = cache_state_[dst_node].find(src);

    // if the data is in our structure we continue

    if (search != cache_state_[dst_node].end()) {

        // now check whether the sizes match
        // TODO: second.size_ >= size would also work

        if (search->second.size_ == size) {
            std::cout << "[+] Found Cached version for 0x" << std::hex << (uint64_t)src << std::dec << std::endl;

            // return a unique copy of the entry which uses the object
            // lifetime and destructor to safely handle deallocation

            return std::move(std::make_unique<CacheData>(search->second));
        }
        else {
            std::cout << "[!] Found Cached version with size missmatch for 0x" << std::hex << (uint64_t)src << std::dec << std::endl;

            // if the sizes missmatch then we clear the current entry from cache
            // which will cause its deletion only after the last possible outside
            // reference is also destroyed

            cache_state_[dst_node].erase(search);
        }
    }

    return nullptr;
}