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Kernel: Add a fast lookup table to the BlockBasedFS disk cache

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Instead of doing a linear scan of the entire cache when doing a lookup,
we now have a nice O(1) HashMap in front of the cache.

The cache miss case can still be improved, this patch really only helps
the cache hit case.

This dramatically improves cached filesystem I/O. :^)
This commit is contained in:
Andreas Kling 2020-11-24 12:08:52 +01:00
parent 20205708b9
commit a6a3c20071
1 changed files with 26 additions and 11 deletions
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37
Kernel/FileSystem/BlockBasedFileSystem.cpp
View file

@ -37,6 +37,7 @@ struct CacheEntry {
u8* data { nullptr };
bool has_data { false };
bool is_dirty { false };
bool is_used { false };
};
class DiskCache {
@ -60,21 +61,25 @@ public:
{
auto now = kgettimeofday().tv_sec;
CacheEntry* oldest_clean_entry = nullptr;
if (auto it = m_map.find(block_index); it != m_map.end()) {
auto& entry = const_cast<CacheEntry&>(entries()[it->value]);
ASSERT(entry.block_index == block_index);
entry.timestamp = now;
return entry;
}
Optional<size_t> oldest_clean_index;
for (size_t i = 0; i < m_entry_count; ++i) {
auto& entry = const_cast<CacheEntry&>(entries()[i]);
if (entry.block_index == block_index) {
entry.timestamp = now;
return entry;
}
ASSERT(!entry.is_used || entry.block_index != block_index);
if (!entry.is_dirty) {
if (!oldest_clean_entry)
oldest_clean_entry = &entry;
else if (entry.timestamp < oldest_clean_entry->timestamp)
oldest_clean_entry = &entry;
if (!oldest_clean_index.has_value())
oldest_clean_index = i;
else if (entry.timestamp < entries()[oldest_clean_index.value()].timestamp)
oldest_clean_index = i;
}
}
if (!oldest_clean_entry) {
if (!oldest_clean_index.has_value()) {
// Not a single clean entry! Flush writes and try again.
// NOTE: We want to make sure we only call FileBackedFS flush here,
// not some FileBackedFS subclass flush!
@ -83,11 +88,20 @@ public:
}
// Replace the oldest clean entry.
auto& new_entry = *oldest_clean_entry;
auto& new_entry = const_cast<CacheEntry&>(entries()[oldest_clean_index.value()]);
// If this entry was occupied, remove the previous mapping from the fast lookup table.
if (new_entry.block_index)
m_map.remove(new_entry.block_index);
// Create a fast lookup mapping from the block index to this entry.
m_map.set(block_index, oldest_clean_index.value());
new_entry.timestamp = now;
new_entry.block_index = block_index;
new_entry.has_data = false;
new_entry.is_dirty = false;
new_entry.is_used = true;
return new_entry;
}
@ -103,6 +117,7 @@ public:
private:
BlockBasedFS& m_fs;
mutable HashMap<u32, u32> m_map;
size_t m_entry_count { 10000 };
KBuffer m_cached_block_data;
KBuffer m_entries;