On Tue, 16 Jun 2020 13:45:27 -0700 Nitin Gupta <nigupta@xxxxxxxxxx> wrote:
> For some applications, we need to allocate almost all memory as
> hugepages. However, on a running system, higher-order allocations can
> fail if the memory is fragmented. Linux kernel currently does on-demand
> compaction as we request more hugepages, but this style of compaction
> incurs very high latency. Experiments with one-time full memory
> compaction (followed by hugepage allocations) show that kernel is able
> to restore a highly fragmented memory state to a fairly compacted memory
> state within <1 sec for a 32G system. Such data suggests that a more
> proactive compaction can help us allocate a large fraction of memory as
> hugepages keeping allocation latencies low.
>
> ...
>
All looks straightforward to me and easy to disable if it goes wrong.
All the hard-coded magic numbers are a worry, but such is life.
One teeny complaint:
>
> ...
>
> @@ -2650,12 +2801,34 @@ static int kcompactd(void *p)
> unsigned long pflags;
>
> trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
> - wait_event_freezable(pgdat->kcompactd_wait,
> - kcompactd_work_requested(pgdat));
> + if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
> + kcompactd_work_requested(pgdat),
> + msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC))) {
> +
> + psi_memstall_enter(&pflags);
> + kcompactd_do_work(pgdat);
> + psi_memstall_leave(&pflags);
> + continue;
> + }
>
> - psi_memstall_enter(&pflags);
> - kcompactd_do_work(pgdat);
> - psi_memstall_leave(&pflags);
> + /* kcompactd wait timeout */
> + if (should_proactive_compact_node(pgdat)) {
> + unsigned int prev_score, score;
Everywhere else, scores have type `int'. Here they are unsigned. How come?
Would it be better to make these unsigned throughout? I don't think a
score can ever be negative?
> + if (proactive_defer) {
> + proactive_defer--;
> + continue;
> + }
> + prev_score = fragmentation_score_node(pgdat);
> + proactive_compact_node(pgdat);
> + score = fragmentation_score_node(pgdat);
> + /*
> + * Defer proactive compaction if the fragmentation
> + * score did not go down i.e. no progress made.
> + */
> + proactive_defer = score < prev_score ?
> + 0 : 1 << COMPACT_MAX_DEFER_SHIFT;
> + }
> }
