Percona Server 5.7 parallel doublewrite

In this blog post, we’ll discuss the ins and outs of Percona Server 5.7 parallel doublewrite.

After implementing parallel LRU flushing as described in the previous post, we went back to benchmarking. At first, we tested with the doublewrite buffer turned off. We wanted to isolate the effect of the parallel LRU flusher, and the results validated the design. Then we turned the doublewrite buffer back on and saw very little, if any, gain from the parallel LRU flusher. What happened? Let’s take a look at the data:


We see that the doublewrite buffer mutex is gone as expected and that the top waiters are the rseg mutexes and the index lock (shouldn’t this be fixed in 5.7?). Then we checked PMP:

Again we see that PFS is not telling the whole story, this time due to a missing annotation in XtraDB. Whereas the PFS results might lead us to leave the flushing analysis and focus on the rseg/undo/purge or check the index lock, PMP clearly shows that a lack of free pages is the biggest source of waits. Turning on the doublewrite buffer makes LRU flushing inadequate again. This data, however, doesn’t tell us why that is.

To see how enabling the doublewrite buffer makes LRU flushing perform worse, we collect PFS and PMP data only for the server flusher (cleaner coordinator, cleaner worker, and LRU flusher) threads and I/O completion threads:


If we zoom in from the whole server to the flushers only, the doublewrite mutex is back. Since we removed its contention for the single page flushes, it must be the batch doublewrite buffer usage by the flusher threads that causes it to reappear. The doublewrite buffer has a single area for 120 pages that is shared and filled by flusher threads. The page add to the batch action is protected by the doublewrite mutex, serialising the adds, and results in the following picture:


By now we should be wary of reviewing PFS data without checking its results against PMP. Here it is:

As with the single-page flush doublewrite contention and the wait to get a free page in the previous posts, here we have an unannotated-for-Performance Schema doublewrite OS event wait (same bug 80979):

This is as bad as it looks (the comment is outdated). A running doublewrite flush blocks any doublewrite page add attempts from all the other flusher threads for the duration of the flush (up to 120 data pages written twice to storage):

dblw_ms_2 (2)

The issue also occurs with MySQL 5.7 multi-threaded flusher but becomes more acute with the PS 5.7 multi-threaded LRU flusher. There is no inherent reason why all the parallel flusher threads must share the single doublewrite buffer. Each thread can have its own private buffer, and doing so allows us to add to the buffers and flush them independently. This means a lot of synchronisation simply disappears. Adding pages to parallel buffers is fully asynchronous:dblw_ps_1

And so is flushing them:


This behavior is what we shipped in the 5.7.11-4 release, and the performance results were shown in a previous post. To see how the private doublewrite buffer affects flusher threads, let’s look at isolated data for those threads again.

Performance Schema:


It shows the redo log mutex as the current top contention source from the PFS point of view, which is not caused directly by flushing.

PMP data looks better too:

The buf_dblwr_flush_buffered_writes now waits for its own thread I/O to complete and doesn’t block other threads from proceeding. The other top mutex waits belong to the LRU list mutex, which is again not caused directly by flushing.

This concludes the description of the current flushing implementation in Percona Server. To sum up, in these post series we took you through the road to the current XtraDB 5.7 flushing implementation:

  • Under high concurrency I/O-bound workloads, the server has a high demand for free buffer pages. This demand can be satisfied by either LRU batch flushing, either single page flushing.
  • Single page flushes cause a lot of doublewrite buffer contention and are bad even without the doublewrite.
  • Same as in XtraDB 5.6, we removed the single page flushing altogether.
  • Existing cleaner LRU flushing could not satisfy free page demand.
  • Multi-threaded LRU flushing design addresses this issue – if the doublewrite buffer is disabled.
  • If the doublewrite buffer is enabled, MT LRU flushing contends on it, negating its improvements.
  • Parallel doublewrite buffers address this bottleneck.

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