On Wed, May 28 2008, Alan D. Brunelle wrote: > Jens Axboe wrote: > > On Tue, May 27 2008, Alan D. Brunelle wrote: > > > >> From 43c8ea2b78f31d7ccd349384a9a2084e787aafc1 Mon Sep 17 00:00:00 2001 > >> From: Alan D. Brunelle <alan.brunelle@xxxxxx> > >> Date: Tue, 27 May 2008 10:32:36 -0400 > >> Subject: [PATCH] Changed blk trace msgs to directly use relay buffer > >> > >> Allows for SMP-usage without corruption, and removes an extra copy at > >> the expense of copying extra bytes. Reduced message size from 1024 to 128. > > > > Or, alternatively, something like the below. Then we don't > > unconditionally reserve and copy 128 bytes for each message, at the > > cost 128 bytes per-cpu per trace. > > I looked into something like this, but thought the added complexity > wasn't worth it. Besides the extra per-cpu stuff, you also have an > extra memcopy involved - in my patch you print directly into the relay > buffer. I figure that /if/ copying (128-msg_size) extra bytes is too > much, one could always shrink the 128 down further. [I would think 64 > bytes is probably ok.] > > I'd bet that the reduced complexity, and skipping the extra memcopy > more than offsets having to copy a few extra bytes... The complexity is the same imho, both versions are fairly trivial. I wasn't out to optimize this in a memory copy sense. To me the most precious resource is the data stream to the app, and 128 bytes is probably 6 times larger than the normal message would be. With the actual trace structure, we are down to about 3 times the byte size. So it was just an idea, I don't care much either way. With 128 bytes, we could just put the buffer on the stack (and still do the copy to the relay buffer). The per-cpu buffers has the advantage that we could grow the size easily if we wanted to. So, given everything, which do you prefer? -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-btrace" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
