Re: slab allocator performance issues

Hi,

On 2021-08-01 19:59:18 +0200, Tomas Vondra wrote:
> In the attached .ods file with results, the "comparison" sheets are the
> interesting ones - the last couple columns compare the main metrics for the
> two patches (labeled patch-1 and patch-2) to master.

I assume with patch-1/2 you mean the ones after the benchmark patch
itself?

> Overall, the results look quite good - patch-1 is mostly on par with master,
> with maybe 5% variability in both directions. That's expected, considering
> the patch does not aim to improve performance.

Not for slab anyway...

> The second patch brings some nice improvements - 30%-50% in most cases (for
> both allocation and free) seems pretty nice. But for the "increase" FIFO
> pattern (incrementally allocating/freeing more memory) there's a significant
> regression - particularly for the allocation time. In some cases (larger
> chunks, block size does not matter too much) it jumps from 25ms to almost
> 200ms.

I'm not surprised to see some memory usage increase some, but that
degree of time overhead does surprise me. ISTM there's something wrong.

It'd probably worth benchmarking the different improvements inside the
WIP: slab performance. patch. There's some that I'd expect to be all
around improvements, whereas others likely aren't quite that clear
cut. I assume you'd prefer that I split the patch up?

> This seems unfortunate - the allocation pattern (FIFO, allocating more
> memory over time) seems pretty common, and the slowdown is significant.

Did you analyze what causes the regressions?

Greetings,

Andres Freund