on_evicted() is invoked in the LSA allocator context, set in the
reclaimer callback instaled by the cache_tracker. However,
cached_pages are allocated in the standard allocator context (note:
page content is allocated inside LSA via lsa_buffer). The LSA region
will happilly deallocate these, thinking that they these are large
objects which were delegated to the standard allocator. But the
_non_lsa_memory_in_use metric will underflow. When it underflows
enough, shard_segment_pool.total_memory() will become 0 and memory
reclamation will stop doing anything, leading to apparent OOM.
The fix is to switch to the standard allocator context inside
cached_page::on_evicted(). evict_range() was also given the same
treatment as a precaution, it currently is only invoked in the
standard allocator context.
Fixes#10056
If memory reclamation is triggered inside _cache.emplace(), the _cache
btree can get corrupted. Reclaimers erase from it, and emplace()
assumes that the tree is not modified during its execution. It first
locates the target node and then does memory allocation.
Fix by running emplace() under allocating section, which disables
memory reclamation.
The bug manifests with assert failures, e.g:
./utils/bptree.hh:1699: void bplus::node<unsigned long, cached_file::cached_page, cached_file::page_idx_less_comparator, 12, bplus::key_search::linear, bplus::with_debug::no>::refill(Less) [Key = unsigned long, T = cached_file::cached_page, Less = cached_file::page_idx_less_comparator, NodeSize = 12, Search = bplus::key_search::linear, Debug = bplus::with_debug::no]: Assertion `p._kids[i].n == this' failed.
Fixes#9915
Message-Id: <20220130175639.15258-1-tgrabiec@scylladb.com>
Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
Currently, reading a page range would issue I/O for each missing
page. This is inefficient, better to issue a single I/O for the whole
range and populate cache from that.
As an optimization, issue a single I/O if the first page is missing.
This is important for index reads which optimistically try to read
32KB of index file to read the partition index page.
Will be needed later for reading a page view which cannot use
make_tracked_temporary_buffer(). Standardize on get_page_units(),
converting existing code to wrap the units in a deleter.
After this patch, there is a singe index file page cache per
sstable, shared by index readers. The cache survives reads,
which reduces amount of I/O on subsequent reads.
As part of this, cached_file needed to be adjusted in the following ways.
The page cache may occupy a significant portion of memory. Keeping the
pages in the standard allocator could cause memory fragmentation
problems. To avoid them, the cache_file is changed to keep buffers in LSA
using lsa_buffer allocation method.
When a page is needed by the seastar I/O layer, it needs to be copied
to a temporary_buffer which is stable, so must be allocated in the
standard allocator space. We copy the page on-demand. Concurrent
requests for the same page will share the temporary_buffer. When page
is not used, it only lives in the LSA space.
In the subsequent patches cached_file::stream will be adjusted to also support
access via cached_page::ptr_type directly, to avoid materializating a
temporary_buffer.
While a page is used, it is not linked in the LRU so that it is not
freed. This ensures that the storage which is actively consumed
remains stable, either via temporary_buffer (kept alive by its
deleter), or by cached_page::ptr_type directly.
It's an adpator between seastar::file and cached_file. It gives a
seastar::file which will serve reads using a given cached_file as a
read-through cache.
We want buffers to be accounted only when they are used outside
cached_file. Cached pages should not be accounted because they will
stay around for longer than the read after subsequent commits.
It is a read-through cache of a file.
Will be used to cache contents of the promoted index area from the
index file.
Currently, cached pages are evicted manually using the invalidate_*()
method family, or when the object is destroyed.
The cached_file represents a subset of the file. The reason for this
is to satisfy two requirements. One is that we have a page-aligned
caching, where pages are aligned relative to the start of the
underlying file. This matches requirements of the seastar I/O engine
on I/O requests. Another requirement is to have an effective way to
populate the cache using an unaligned buffer which starts in the
middle of the file when we know that we won't need to access bytes
located before the buffer's position. See populate_front(). If we
couldn't assume that, we wouldn't be able to insert an unaligned
buffer into the cache.
