On Thu, Jun 29, 2017 at 10:06:52AM -0500, Josh Poimboeuf wrote: > On Thu, Jun 29, 2017 at 04:46:18PM +0200, Ingo Molnar wrote: > > > > * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > > > > > Plus, shouldn't we use __packed for 'struct undwarf' to minimize the > > > > structure's size (to 6 bytes AFAICS?) - or is optimal packing of the main > > > > undwarf array already guaranteed on every platform with this layout? > > > > > > Ah yes, it should definitely be packed (assuming that doesn't affect performance > > > negatively). > > > > So if I count that correctly that should shave another ~1MB off a typical ~4MB > > table size? > > Here's what my Fedora kernel looks like *before* the packed change: > > $ eu-readelf -S vmlinux |grep undwarf > [15] .undwarf_ip PROGBITS ffffffff81f776d0 011776d0 0012d9d0 0 A 0 0 1 > [16] .undwarf PROGBITS ffffffff820a50a0 012a50a0 0025b3a0 0 A 0 0 1 > > The total undwarf data size is ~3.5MB. > > There are 308852 entries of two parallel arrays: > > * .undwarf (8 bytes/entry) = 2470816 bytes > * .undwarf_ip (4 bytes/entry) = 1235408 bytes > > If we pack undwarf, reducing the size of the .undwarf entries by two > bytes, it will save 308852 * 2 = 617704. > > So the savings will be ~600k, and the typical size will be reduced to ~3MB. Just for the record, while packing the struct from 8 to 6 bytes did save 600k, it also made the unwinder ~7% slower. I think that's probably an ok tradeoff, so I'll leave it packed in v3. -- Josh -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
