From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59626) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bCUuB-0000aK-0C for qemu-devel@nongnu.org; Mon, 13 Jun 2016 12:40:04 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bCUu6-00040G-Mp for qemu-devel@nongnu.org; Mon, 13 Jun 2016 12:40:01 -0400 Received: from roura.ac.upc.es ([147.83.33.10]:49271) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bCUu6-0003z4-BT for qemu-devel@nongnu.org; Mon, 13 Jun 2016 12:39:58 -0400 From: =?utf-8?Q?Llu=C3=ADs_Vilanova?= References: <145641255678.30097.2919142707547689749.stgit@localhost> <145641258612.30097.7127731954660712163.stgit@localhost> <30b46b30-fbc0-31a4-6210-657b205c121f@redhat.com> <87lh29cmip.fsf@fimbulvetr.bsc.es> Date: Mon, 13 Jun 2016 18:39:46 +0200 In-Reply-To: (Paolo Bonzini's message of "Mon, 13 Jun 2016 16:08:50 +0200") Message-ID: <878ty9xctp.fsf@fimbulvetr.bsc.es> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH 4/6] trace: Add per-vCPU tracing states for events with the 'vcpu' property List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: qemu-devel@nongnu.org, Blue Swirl , Riku Voipio , Eduardo Habkost , Stefan Hajnoczi , Andreas =?utf-8?Q?F=C3=A4rber?= Paolo Bonzini writes: > On 13/06/2016 14:15, Llu=C3=ADs Vilanova wrote: >> > That said, I am skeptical about the benefit of the interfaces you are >> > adding. They add a lot of complication and overhead (especially >> > regarding the memory/cache overhead of the dstate array) without a cle= ar >> > use case, in my opinion; all the processing you do at run-time is just >> > as well suited for later filtering. >>=20 >> This should make tracing faster on the future with multi-threaded TCG, a= s well >> as trace files much smaller if you're tracing something like memory >> accesses. Also, bear in mind this series was split from a much larger on= e for >> simplicity. The follow-up one provides much larger performance benefits = by >> avoiding the generation of TCG code to call the tracing backend when a v= CPU is >> not traced. > This still assumes that tracing only some VCPUs is a common use case. > Is it?... I use it for code profiling by sampling across vCPUs, or only on vCPUs I kn= ow run processes of my interest. The profiles can then be used for analyzing t= he application/system behaviour. Also, with the fast-path checks already in place ('trace_events_enabled_cou= nt'), performance when not tracing should never be worse with this series. If this feature does not look useful to overall QEMU I will fold it into my other out-of-tree patches. Cheers, Lluis