Messages in this thread | | | Date | Tue, 17 Sep 2013 20:49:51 -0400 | From | Peter Hurley <> | Subject | Re: increased vmap_area_lock contentions on "n_tty: Move buffers into n_tty_data" |
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On 09/12/2013 09:09 PM, Fengguang Wu wrote: > On Fri, Sep 13, 2013 at 08:51:33AM +0800, Fengguang Wu wrote: >> Hi Peter, >> >> FYI, we noticed much increased vmap_area_lock contentions since this >> commit: >> >> commit 20bafb3d23d108bc0a896eb8b7c1501f4f649b77 >> Author: Peter Hurley <peter@hurleysoftware.com> >> Date: Sat Jun 15 10:21:19 2013 -0400 >> >> n_tty: Move buffers into n_tty_data >> >> Reduce pointer reloading and improve locality-of-reference; >> allocate read_buf and echo_buf within struct n_tty_data. > > Here are some comparison between this commit [o] with its parent commit [*].
[...]
> 8cb06c983822103da1cf 20bafb3d23d108bc0a89 > ------------------------ ------------------------ > 976.67 +108.3% 2034.67 lkp-a04/micro/netperf/120s-200%-TCP_STREAM > 8971.36 +11.4% 9997.05 nhm-white/micro/aim7/exec_test > 9948.03 +20.9% 12031.72 TOTAL slabinfo.kmalloc-128.active_objs > > 8cb06c983822103da1cf 20bafb3d23d108bc0a89 > ------------------------ ------------------------ > 976.67 +108.3% 2034.67 lkp-a04/micro/netperf/120s-200%-TCP_STREAM > 9127.64 +11.4% 10164.15 nhm-white/micro/aim7/exec_test > 10104.31 +20.7% 12198.82 TOTAL slabinfo.kmalloc-128.num_objs
The dramatic increase in 128-byte kmalloc blocks is from vmalloc overhead with associated with each allocation. On a x86_64, struct vmap_area is 104 bytes, rounded to 128, allocated with every vmalloc allocation. This is approx 1% overhead (which seems high to me).
The reason this is still visible after the test completes is the vmap area is lazily reclaimed (see mm/vmalloc.c:__purge_vmap_area_lazy()).
1% memory overhead coupled with the unwanted vmap_area_lock contention (even though it is test-induced) -- I might revert this anyway.
Regards, Peter Hurley
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