A Hierarchical Object Cache
USC and UC Boulder; Armando's comments on lessons learned and how they apply
to TCS proxy
Various important (and nonobvious) lessons from implementing Harvest cache.
- Detailed timing breakdown shows TCP setup is responsible for 15ms
of the 20ms Harvest response time seen by clients.
- Getting the nonblocking disk I/O and select loop right required tuning
for different systems.
- Transparency was hardest to get right, due to initial assumption that
URL plus MIME headers gives unique object name; MIME headers are
vastly different for different clients.
- Timeouts from large MIME headers that required fragmenting sometimes
caused nonmaskable faults to user.
- Some noncompliant http servers close client connection before reading
all MIME headers!
- DNS "negative caching" timeouts were too lengthy, causing
users to report that DNS lookups worked fine until Harvest was used.
- Browser-specific dynamically-generated Web pages cause problems with
hit rates and really require MIME headers to be included in comparison for
- Client and server implementation differences, noncompliance with standards,
and vendor interoperability in general have forced tradeoffs between efficiency/performance,
design cleanliness and operational transparency.
- Keeping metadata in memory and limiting the VM image size to avoid
page faults was an important win.
- Monolithic filesystems are the wrong model for the evolving Internet:
feature set is overkill for many applications, implementations are complex
and nonmodular, vendor interoperability is more difficult since components
are "larger" and more tightly coupled to rest of OS...
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