Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)
Hi Avi, On Mon, 2010-01-18 at 14:01 +0200, Avi Kivity wrote: Maybe you place no value on uprobes. But people who debug userspace likely will see a reason. On 01/18/2010 02:06 PM, Peter Zijlstra wrote: I do see value in uprobes, I just don't like it mucking about with the address space. Nor does it appear required. On Mon, Jan 18, 2010 at 2:09 PM, Avi Kivity a...@redhat.com wrote: Well, the alternatives are very unappealing. Emulation and single-stepping are going to be very slow compared to a couple of jumps. So how big chunks of the address space are we talking here for uprobes?
Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)
On Mon, Jan 18, 2010 at 2:44 PM, Srikar Dronamraju sri...@linux.vnet.ibm.com wrote: * Avi Kivity a...@redhat.com [2010-01-18 14:17:10]: On 01/18/2010 02:13 PM, Pekka Enberg wrote: So how big chunks of the address space are we talking here for uprobes? That's for the authors to answer, but at a guess, 32 bytes per probe (largest x86 instruction is 15 bytes), so 32 MB will give you a million probes. That's a piece of cake for x86-64, probably harder to justify for i386. On x86, each probe takes 16 bytes. And how many probes do we expected to be live at the same time in real-world scenarios? I guess Avi's one million is more than enough?
Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)
On 01/18/2010 02:51 PM, Pekka Enberg wrote: And how many probes do we expected to be live at the same time in real-world scenarios? I guess Avi's one million is more than enough? Avi Kivity kirjoitti: I don't think a user will ever come close to a million, but we can expect some inflation from inlined functions (I don't know if uprobes replicates such probes, but if it doesn't, it should). Right. I guess we're looking at few megabytes of the address space for normal scenarios which doesn't seem too excessive. However, as Peter pointed out, the bigger problem is that now we're opening the door for other features to steal chunks of the address space. And I think it's a legitimate worry that it's going to cause problems for 32-bit in the future. I don't like the idea but if the performance benefits are real (are they?), maybe it's a worthwhile trade-off. Dunno. Pekka
