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#pragma once
#include <map>
#include <mutex>
#include <Core/Types.h>
#include <IO/OpenedFile.h>
#include <Common/ElapsedTimeProfileEventIncrement.h>
#include <Common/ProfileEvents.h>
#include <city.h>
namespace ProfileEvents
{
extern const Event OpenedFileCacheHits;
extern const Event OpenedFileCacheMisses;
extern const Event OpenedFileCacheMicroseconds;
}
namespace DB
{
/** Cache of opened files for reading.
* It allows to share file descriptors when doing reading with 'pread' syscalls on readonly files.
* Note: open/close of files is very cheap on Linux and we should not bother doing it 10 000 times a second.
* (This may not be the case on Windows with WSL. This is also not the case if strace is active. Neither when some eBPF is loaded).
* But sometimes we may end up opening one file multiple times, that increases chance exhausting opened files limit.
*/
class OpenedFileCache
{
class OpenedFileMap
{
using Key = std::pair<std::string /* path */, int /* flags */>;
using OpenedFileWeakPtr = std::weak_ptr<OpenedFile>;
using Files = std::map<Key, OpenedFileWeakPtr>;
Files files;
std::mutex mutex;
public:
using OpenedFilePtr = std::shared_ptr<OpenedFile>;
OpenedFilePtr get(const std::string & path, int flags)
{
Key key(path, flags);
std::lock_guard lock(mutex);
auto [it, inserted] = files.emplace(key, OpenedFilePtr{});
if (!inserted)
{
if (auto res = it->second.lock())
{
ProfileEvents::increment(ProfileEvents::OpenedFileCacheHits);
return res;
}
}
ProfileEvents::increment(ProfileEvents::OpenedFileCacheMisses);
OpenedFilePtr res
{
new OpenedFile(path, flags),
[key, this](auto ptr)
{
{
std::lock_guard another_lock(mutex);
files.erase(key);
}
delete ptr;
}
};
it->second = res;
return res;
}
void remove(const std::string & path, int flags)
{
Key key(path, flags);
std::lock_guard lock(mutex);
files.erase(key);
}
};
static constexpr size_t buckets = 1024;
std::vector<OpenedFileMap> impls{buckets};
public:
using OpenedFilePtr = OpenedFileMap::OpenedFilePtr;
OpenedFilePtr get(const std::string & path, int flags)
{
ProfileEventTimeIncrement<Microseconds> watch(ProfileEvents::OpenedFileCacheMicroseconds);
const auto bucket = CityHash_v1_0_2::CityHash64(path.data(), path.length()) % buckets;
return impls[bucket].get(path, flags);
}
void remove(const std::string & path, int flags)
{
ProfileEventTimeIncrement<Microseconds> watch(ProfileEvents::OpenedFileCacheMicroseconds);
const auto bucket = CityHash_v1_0_2::CityHash64(path.data(), path.length()) % buckets;
impls[bucket].remove(path, flags);
}
static OpenedFileCache & instance()
{
static OpenedFileCache res;
return res;
}
};
using OpenedFileCachePtr = std::shared_ptr<OpenedFileCache>;
}
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