Messages in this thread | | | Date | Tue, 13 May 2003 10:25:56 -0400 | From | Timothy Miller <> | Subject | Re: hammer: MAP_32BIT |
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H. Peter Anvin wrote: > Timothy Miller wrote: > >>>The purpose is that there is a slight task-switching speed advantage if >>>the address is in the bottom 4 GB. Since this affects every process, >>>and most processes use very little TLS, this is worthwhile. >>> >>>This is fundamentally due to a K8 design flaw. >> >>Is there an explicit check somewhere for this? Are the page tables laid >>out differently? >> > > > No, there are two ways to load the FS base register: use a descriptor, > which is limited to 4 GB but is faster, or WRMSR, which is slower, but > unlimited. >
Ulrich Drepper wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > Timothy Miller wrote: > > >>Why does there ever need to be an explicit HINT that you would prefer a >><32 bit address, when it's known a priori that <32 is better? Why >>doesn't the mapping code ALWAYS try to use 32-bit addresses before >>resorting to 64-bit? > > > Because not all memory is addressed via GDT entries. In fact, almost > none is, only thread stacks and similar gimicks. If all mmap memory > would by default be served from the low memory pool you soon run out of > it and without any good reason.
All I have to say is... I appreciate your patience with my ignorant questions. :)
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